History of Psychology 2014. Vol. 17, No.2, 149-158
© 2014 American Psychological Association 1093-4510/14/$12.00 DOi: 10.IO37/a0O35332
SCHUMANN'S WHEEL TACHISTOSCOPE: Its Reconstruction and Its Operation Armin Stock University of Wuerzburg In the fall and winter of 1910, Max Wertheimer (1880-1943) performed his famous experiments on perceived motion, published in 1912. Besides slider experiments he mainly used a wheel tachistoscope developed by Friedrich Schumann (1863-1940) at the end of the 19th century. The Adolf-Wuerth-Center for the History of Psychology has several wheel tacbistoscopes in its collection of instruments. Their provenance can be traced back to the Institute of Psychology of the University of Frankfurt and the University of Zurich. It is very plausible that Wertheimer, who performed his experiments at the Frankfurt Institute, used one of them. But the wheel tachistoscope alone is not sufficient to reconstruct Wertheimer's original experiments. As always, the devil is in the details. Wertheimer's descriptions of the necessary accessories, a prism, a viewing device, and an electric motor to move the wheel, are rather sparse. This article describes the results of a search for traces in the literature, in archives, and in literary depositories to shed some light on Wertheimer's experimental equipment. As a result, it was possible to reconstruct the entire apparatus and to obtain the same optical impressions with the reconstructed devices as Wertheimer's observers reported. In addition, one of his results was replicated with new participants exactly 100 years after its first publication. Keywords: wheel tachistoscope, phi phenomenon, Friedrich Schumann, Max Wertheimer, apparent motion
The 61st volume of the Zeitschrift für Psychologie {Journal of Psychology) edited by Friedrich Schumann in 1912 consists of a total of six papers. Two of them deal with aspects of perceived motion and were written at the Institute of Psychology of the Frankfurt Academy directed by Schumann, which is the forerunner of the University of Frankfurt. For the first—and rather unknown—paper by Woldemar Lasersohn, entitled "Critique of the Main Theories of the Immediate Impression of Motion," Schumann was the instigator. About the second, a paper that became internationally
I am very grateful to Michael Wertheimer for many suggestions for improvement.s to an earlier version of this article. In addition, I thank the Adolf Wuerth GmbH & Co. KG for supporting the Adolf-Wuerth-Center for the History of Psychology. Correspondence concerning this article should be addressed to Armin Stock, Adolf-Wuerth-Center for the History of Psychology, University of Wuerzburg, Pleicherwall 1, D-97070 Würzburg, Germany. E-mail; Armin. [email protected]
recognized, titled "Experimental Studies on Seeing Motion" and written by private lecturer Dr. Max Wertheimer, Schumann said only: ". . . a fairly independent study about the same topic . . . " (Schumann in Lasersohn, 1912, p. 81). Schumann described it the same way at the Fifth Congress for Experimental Psychology in Berlin on the April 18, 1912 (Schumann, 1912, p. 181). Schumann, however, also wanted to study this issue himself and had already instructed a Mr. Chaym to conduct preliminary studies at the Psychological Institute in Berlin, many years before Wertheimer's research (cf. Lasersohn, 1912, p. 102). Therefore, a suitable experimental procedure had already been prepared and passed on to Wertheimer some years later, who then also became interested in questions related to the perception of motion (cf. Lasersohn, p. 81). The aim of our project was to reconstruct the apparatus and to use Schumann's experimental approach that he recommended to Wertheimer. A key component of this experimental approach is the wheel tachis149
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toscope developed by Schumann. Hence, it became necessary to go back into the 19th century and determine when Schumann constructed such an instrument and how it worked. Friedrich Schumann: His Life and the Development of His Wheel Tachistoscope Friedrich Schumann (Figure 1) was bom in Hildesheim near Hannover on June 16, 1863. In school he demonstrated a special talent for mathematics, as did Max Wertheimer (cf. Metzger, 1940, p. 2). This may have been one reason for Schumann's decision to study phys-
Figure I.
ics at the University of Goettingen. He began his studies in 1881 and finished them in 1885 with a mathematics-based thesis on "Electromagnetic rotation-phenomena of liquid conducting mediums" (Schumann, 1887). After that, he focused on psychological research and became an assistant to Georg Elias Mueller (1850-1934) in Goettingen. In 1892, he finished his habilitation work "On the estimation of small amounts of time." This paper (1893) developed a field of research important for the later construction of the wheel tachistoscope. From 1894 to 1905, Schumann was the first assistant to Carl Stumpf (1848-1936) in Berlin.
Friedrich Schumann (1863-1940) (cf. Schumann & Katz, 1933, p. I).
SCHUMANN'S WHEEL TACHISTOSCOPE
Late in this time period he made the acquaintance of the young student Max Wertheimer. In 1905, he was called to a chair at the University of Zurich, which he abandoned 5 years later in favor of a professorship at the Psychological Institute of the Frankfurt Academy as successor to Karl Marbe (1869-1953). Among his first assistants were Wolfgang Koehler (1887-1967) and Kurt Koffka (1886-1941). In the same year (1910), Max Wertheimer (1880-1943) became one of his assistants as well. With these young researchers, Frankfurt developed into the key location for the beginning of Gestalt Psychology. Schumann served as head of the Frankfurt Institute until 1928. In 1929, Wertheimer became bis successor. Schumann died in Frankfurt in 1940. Schumann developed his wheel tachistoscope during his years as assistant to Carl Stumpf in Berlin. Wertheimer used it for his studies of apparent motion, the so-called phi phenomenon. It is unclear exactly when Schumann began the construction of the wheel tachistoscope. The first hint of an existing functional instrument of this kind can be found in 1899, when officers of the psychological society in Berlin, its chairman Flatau and its secretary Giering, visited the psychological institute of Professor Stumpf. The tour through the Berlin Institute was among others conducted by private lecturer Dr. Schumann, who took the opportunity to introduce his visitors to his newly developed wheel tachistoscope. This instrument allows " . . . making objects visible within the field of vision of a telescope for a short period of time. It makes it possible to erase the afterimage after a variable and precisely measurable amount of time through intense illumination of the field of vision" (Flatau & Giering, 1899, p. 97). Furthermore, the instrument makes it possible for ". . . two objects to become visible after each other at any interval" (p. 98). Hence, it can control all conditions needed to produce apparent motion. A detailed description of the wheel tachistoscope exists in a report on an exhibit of experimental psycbological instruments and methods at the first Congress for Experimental Psychology in Gießen in 1904 (Sommer, 1904, p. 14). The tachistoscope with electric motor, by Schumann (Berlin), was exhibit number two. The following is an excerpt from its description;
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A wheel of approx. [3/4] m diameter is carried on an axis running on bearings . . . . The periphery of the wheel is a 10 cm wide metal strip which can be divided into 8 parts. One of these octants carries a gap of variable width and a neighboring octant an adjustable mirror, inclined 45° from the plane of the wheel. A telescope is attached to the instrument such that its field of vision is masked by the iron ring and only uncovered for a moment when the gap at the objecüve passes by . . . . The instrument is set in motion via the pulley of a small electric motor. Its rotation speed is held constant by a von Helmholtz centrifugal regulator. (Sommer, 1904, p. 14f.)
A footnote to the description refers to the producer of the tachistoscope, the Spindler & Hoyer Company in Goettingen. Figure 2 was printed in the company's catalog for 1908 (Spindler & Hoyer, 1908, p. 168). For the use of this tachistoscope, the company offered to supply a reading telescope (Figure 3) with a close focus area of less than 1 m (Spindler & Hoyer, 1908, p. 170). Schumann spoke twice about his wheel tachistoscope at the second Congress for Experimental Psychology in Wuerzburg in 1906. In addition to a description of the advantages of rotating tachistoscopes against falling ones (Schumann, 1907, p. 164), he indicated that he had improved his wheel tachistoscope for rapidly succeeding exposures of different objects; tbe new model was produced by tbe Zulauf & Co Company in Zurich (Schumann, 1907, p. 165). Additional evidence that a new and an old model of Schumann's tachistoscope existed is Koffka's (1931, p. 1170) statement that the "old model" of Spindler & Hoyer is pictured in Klemm's (1925, p. 93) work. A more important footnote stated that Schumann had already conducted tachistoscopic apparent movement experiments. He called the content of awareness, the perceived motion which appeared, in accordance with von Ehrenfels "Gestalt quality" (Schumann, 1907, p. 218). Wertheimer's Use of Schumann's Wheel Tachistoscope The well-known picture of Max Wertheimer from 1913 (cf. King & Wertheimer, 2005; Spillmann, 2012), where he is next to a wheel tachistoscope (Figure 4), shows that Wertheimer did not work with the tachistoscope produced by Spindler and Hoyer, but with the newer Zurich model. The Zurich model is different from the old model in its spoke structure and in
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m Figure 2. Drawing of the Schumann wheel tachistoscope in the catalog of the Spindler & Hoyer Company, Goettingen (1908. p. 168). The octants are separated by eight spokes.
its large cast-iron fly wheel. The structure of the outer ring, which is not divided into octants as in the old model, is also clearly different. Instead, there are viewing windows that can be opened and closed via shutters. The instrument Wertheimer used could not have been built in Frankfurt because of time constraints: Schumann was called to Frankfurt for the 1910 summer term, and Wertheimer already conducted his experiments during the autumn and winter of 1910 (cf. Wertheimer, 1912, p. 175). Hence, Schumann probably brought the instrument with him from Zurich to Frankfurt. In addition to the tachistoscope, Wertheimer needed a viewing scope for his experiments, as well as a prism and an electric motor. He named all these items briefly, but did not describe them in detail: The Schumann apparatus . . . has a prism close to the tachistoscopic wheel, beyond the distal lens of the telescope one looks through. This prism blocks the lower half of the lens, so that rays come into the upper half of the lens directly, while entering the lower half from the side. One exposure slit on the wheel exposes the upper half, whereas a second slit exposes the lower half. If the distance between the lens and the prism is small, then the whole circular surface of each momentary exposure field is seen at each of the two exposures. . . . The distance of the exposure fields from the prism
was about 80 cm. (Wertheimer, 1912, pp. 175. 177; Translation from Spillmann, 2012, p. 13f.)
A detailed examination of the 1913 picture gives the impression that Wertheimer is holding something in his hands. At first glance it seems to be a cigarette between the thumb and the forefinger of his right hand. But there are several reasons against this first assumption. The picture appears to be composed with the intent of showing Wertheimer with his apparatuses. In addition, a cigarette would most likely not have been accepted by the photographer, and we cannot detect any smoke in the picture. Finally, Wertheimer would have burned his left forefinger keeping a cigarette in this position. Another consideration must be kept in mind as well: The object seems to be almost transparent and because it is physically not possible to bend only two of the three joints of the left forefinger without pressing the forefinger against an object, Wertheimer was not holding it in his right but in his left hand. The straight finger position of the first two joints of Wertheimer's left forefinger gives us a hint about the length of the object: The distance between the fingertip and the second joint of a male adult's index finger tends to be between 4 and 5 cm in length.
SCHUMANN'S WHEEL TACHISTOSCOPE
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Figure 3. Drawing of the reading telescope available together with the wheel tachistoscope in the catalogue of the Spindler & Hoyer Company, Goettingen (1908, p. 170).
Hence, we can speculate that the object has a similar length. Finally two more physical observations give us further information about the nature of the object; Because there are no hard shadows, we can conclude that the photographer did not use a flash light. Instead he could have used two lamps: One lamp on the left side produced a small shadow of the stand on the cast iron wheel of the tachistoscope. The other lamp was opposite to Wertheimer's head to illuminate his face. This second lamp brightened up the stimuli on the table, as well as caused a slight reflection of the object in Wertheimer's hands. Finally, we can see a duplication of Wertheimer's left thumb, which is partly below the object. All these considerations lead to the conclusion that the object in Wertheimer's hand was the prism that he required for his experiments, and that it was about 4-5 cm long. Today's prisms often have an enhanced optic quality. However, Wertheimer could not have
used this technical improvement because it was not available before the 1920s. This information comes from the Qioptiq Company, today's successor company to the Spindler & Hoyer Company. It is still quite probable, however, that Wertheimer used a high-quality prism because it is virtually impossible to get a good reflective projection of a collaterally positioned object with a low-quality one. Wertheimer does not present any details about the viewing scope he used. Because of a distance of only 80 cm between the exposure fields and the prism, it can be concluded that a telescope with a low near focal point was needed. This was quite rare at the beginning of the 20th century. Wertheimer may have used the scope sold with the old tachistoscope model produced by Spindler and Hoyer (Figure 3). This scope was capable of producing a sharp image of objects at a distance of less than one meter. Another hint occurs in a dissertation by Julius Wagner (1918). He conducted studies on
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Figure 4. Max Wertheimer next to Schumann's taehistoscope (depicted from the back). Photo taken 1913 (cf. King & Wertheimer, 2005; Spillmann, 2012).
the psychology of reading at Schumann's Institute, using the wheel tachistoscope. Wagner wrote that the institute had two scopes available: a "Zeiss Triëder telescope" with a magnification of 2.5, as well as a smaller astronomic telescope (cf. Wagner, 1918, p. 11). Wagner attached the scope to the middle of the stand of the wheel tachistoscope via a tap hole. However, an inquiry to the archive of the Zeiss Company in Jena made clear that Zeiss did not distribute this kind of telescope in 1918 (e-mail message, Carl Zeiss archive, March 8, 2012). Triëder telescopes were only produced by the Goerz Company in Berlin, which later became a part of the Zeiss Ikon AG, but this company consolidation had not yet happened in 1918. A document corroborating the information of the
Zeiss archive exists among psychologist Friedrich Sander's (1889-1971) papers in our Center. Sander was professor of psychology at the University of Giessen between 1929 and 1933, and the photo in Figure 5 was taken there. The photo shows a Schumann wheel tachistoscope together with an electric motor and a Triëder telescope from the Goerz Company attached on a stand on the right side. The AdolfWuerth-Center owns the electric motor, controlled via cone pulleys and a rheostat, as well as the Triëder telescope. Thus, together with the wheel tachistoscope that came from Frankfurt, it became possible to reconstruct the complete apparatus used by Wertheimer (Figure 6). The last required components to repeat Wertheimer's studies were the objects to be
SCHUMANN'S WHEEL TACHISTOSCOPE
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Figure 5, Photo of a wheel tachistoscope. The picture was taken at the Institute of Psychology of the University of Gießen between 1929 and 1933. In addition to the wheel tachistoscope, the electric motor as well as the Triëder telescope from the Goerz Company can be recognized. (Collection AWZ Würzburg)
presented. Wertheimer proposed that they should be presented on black exposure fields in form of white or colored stripes (cf. Wertheimer, 1912, p. 176). Because the original objects were not in our collection, our restoration workshop produced appropriate replicas. Following Wertbeimer's description we used white stripes of 1 X 6 cm length and illuminated them laterally with a small photo lamp (cf. Wertheimer, 1912, p. 177). Replication of Wertheimer's Apparatus and Procedure The reconstruction of the historic apparatus and procedure went as follows: First, the wheel tachistoscope was placed on a sufficiently wide and long table. For safety reasons, it was securely attached with two bar clamps. Tbe electric motor was placed to the left of the tachistoscope and was connected to the flywheel of the tachistoscope via a belt. Unfortunately, it was not possible to use tbe historic electric motor mentioned above because the risk of damaging it was too high. Instead, we used a new three-phase induction motor that can be
accurately adjusted. In front of the lower middle part of the outer crest of the tachistoscope, a viewing scope was adjusted toward both windows of the wheel flange, such that the stimulus 80 cm behind the tachistoscope could be seen through the upper window, and the stimulus placed at a distance of 80 cm to the right of the tachistoscope could be seen through the lower window. To make the latter object visible through the lower half of the scope, a reflecting 90°-prism with an edge length of 50 X 50 mm from the Qioptiq Company was placed directly opposite the scope on an adjustable stand, as close as possible behind the tachistoscope's outer wheel flange. A certain amount of practice was needed to adjust all elements of the apparatus so as to produce an optimal impression of motion. In addition, this adjustment procedure had to be conducted anew for every test person, and under certain circumstances a new adjustment was also required within a given test series. The Goertz Triëder scope in our collection would have been suitable for the replication of Wertheimer's experiments despite its age. But we did not want to take the risk of damaging it
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Figure 6. Reconstruction of the apparatus shown in Figure 5. This reconstruction was achieved by using items in the collection of the Adolf-Wuerth-Center.
and therefore used a new macroscope from the Minox Company (MS 8 X 25). It was attached directly to the tachistoscope's stand. But the rotation of the tachistoscope led to slight vibrations at higher speeds, which distorted the resulting image impression. Therefore, we decoupled the scope and the tachistoscope and placed the scope at the same distance on a tripod in front of the tachistoscope. Calculation of Exposure Times The exposure times of the two stimuli were determined by the speed of the wheel tachistoscope rotation and by the opening of the exposure fields in degrees. Wertheimer used a stop watch for measuring the time needed for 20 wheel rotations. For example this could have taken 20.4 s. This time divided by 20 yields 1,020 ms for a single 360° rotation of tbe wheel. If the opening of the exposure fields is for example, 7°, the exposure time is (1020/360) X 7 = 19.83 ms per exposure field (cf. Wertheimer, 1912, p. 178). In our experiments, we determined the rotation speed of the wheel tachistoscope with a modern laser analyzer in revolutions per minute (rpm) and calculated the exposure times correspondingly.
Replication of Key Results of Wertheimer's 1912 Publication Wertheimer conducted his research with only three observers—a relatively small sample by today's standards. He wrote: "It turned out that a large number of observers was not necessary since the characteristic phenomena were altogether unequivocal, spontaneous and compelling" (Wertheimer, 1912, p. 177; Translation from Spillmann, 2012, p. 14). His observers were Dr. Wolfgang Koehler, Dr. Kurt Koffka, and Dr. Mira Klein-Koffka. His participants had an average age of 24 years because Kurt Koffka and Mira Klein-Koffka were both born in 1886 and Wolfgang Koehler in 1887. Wertheimer presented his results for the three distinctive perceived impressions on pages 179 and 180 in two tables. These three impressions are succession, optimal motion, and simultaneity. In one of his experiments, two white stripes on a black background with a common vertex at an angle of 45° were presented in the two exposure fields. The difference between Wertheimer's first and second table of results concerns only a variation in the size of the exposure fields. He concluded that not the size of the exposure fields but the time span of the interval t between the exposure fields is essen-
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Table 1 Results of the Replication of Wertheimer's Research After 100 Years With the Schumann Tachistoscope Wertheimer (1912, p. 179f)
Stock (2012)
Interval t(ms)
M
SD
n
M
SD
n
Succession Ideal motion Simultaneity
165,50 57,87 32,66
12,50 9,66 1,59
2 8 6
217,09 56,74 34,52
26,04 2,35 1,80
4 4 4
Note. Although Wertheimer only used the data of three participants, partly higher data can be found in the column "n." They indicate how many values of the three participants were used for the calculation of the arithmetic means.
tial for the impression of motion (Wertheimer, 1912, p. 179). We used openings of 7° and adjusted the interval t to 16° (Wertheimer, 1912, p. 178). Table 1 shows the results found for the three distinctive states of motion for both: Wertheimer's data and the data of our replication. The two sets of data are quite comparable, even though the average age of our four participants was (at nearly 34 years) higher than the average age of the participants in Wertheimer's original experiment. Apparently age is not an important variable in these studies. These results show a striking correspondence between the results of Wertheimer's experiments and our replication for the states of ideal motion and simultaneity. Perhaps for the succession state our observers might have used a stricter criterion, which may have led to the longer intervals t in our replication compared with Wertheimer's original results. However, our replication of Wertheimer's experiments of 100 years ago demonstrates that even today the same results can be produced using Schumann's original wheel tachistoscope. A video documenting the three states of movement has been prepared: It is available on the Internet pages of the Adolf-Wuerth-Center.' Summary and Discussion The project to reconstruct and use one of Schumann's historic tachistoscopes in our collection, which Wertheimer probably worked with in autumn/winter 1910, was successfully completed. One challenge was to find out what kind of scopes could work in the experiments with the wheel tachistoscope and what kind of reflecting prism was used by Wertheimer. Research in the literature, in archives, and in literary depositories yielded suggestions that experiments with wheel tachistoscopes were
carried out with the Triëder telescope manufactured by the Goertz Company or with a reading telescope offered by Spindler and Hoyer. We estimated the size of the reflecting prism from an object apparently held by Wertheimer in a well-known 1913 photograph (Figure 4). The length of the prism appears to be about the length of the first two joints of his left forefinger, which probably is about 4-5 cm. Hence, we used a prism with an edge length of 5 cm. It is unlikely that Wertheimer used a smaller prism because it would not have been possible to produce a good separation between the upper and the lower exposure fields with the window size of the tachistoscope with a smaller prism. A larger prism would not have been of any advantage for Wertheimer and would have been significantly more expensive. It turned out that using the wheel tachistoscope required a certain amount of practice for both experimenter and participant. Aside from such technical challenges as the adjustment of the viewing scope, the prism, and the stimuli and the regulation of the motor speed via cone pulleys and a rheostat, there were significant challenges for the participants who needed a good ability to observe and concentrate. They also needed to be able to direct their attention in particular ways when instructed to do so. Furthermore, the seating position when looking through the scope in direct proximity in front of a rotating disk was uncomfortable. Unfortunately, Wertheimer did not report whether he measured the distinctive impressions (succession, ideal motion, simultaneity) with increasing or decreasing speed of rotation. ' http://www.awz.uni-wuerzburg.de/archiv/film_foto_ tonarchiv/filmdokumente/max_wertheimer_und_das_ schumannsche_tachistoskop/tachistocope_in_action/
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Doubtless the use of a method of limits or adjustment could have increased the precision of measurement. Finally, it must be mentioned that a literature search revealed that Wertheimer carried out experiments that were prethought and prepared by Schumann. He then habilitated with his report on these experiments (cf. Gundlach's contribution in this issue). Wertheimer also did not use the tachistoscope distributed by Spindler and Hoyer, but an improved model built by the Swiss company Zulauf and Co following Schumann's guidelines. Nevertheless, Wertheimer earns credit for having drawn conclusions from his experiments that were meaningful for his time. His studies played a major role in the development and expansion of Gestalt psychology. References Flatau, T. S., & Giering, H. (1899). Psychologischer Verein zu Berlin. Sitzungsberichte [Psychological Society Berlin. Meeting reports]. Zeitschrift für Pädagogische Psychologie, I, 95-100. King, D. B., & Wertheimer, M. (2005). Max Wertheimer and gestalt theory. New Brunswick, NJ: Transaction Publishers. Klemm, O. (1925). Wahmehmungsanalyse [Perception analysis]. In E. Abderhalden (Ed.), Handbuch der biologischen Arbeitsmethoden [Manual of biological working methods] (pp. 1-106). Beriin, Germany: Urban & Schwarzenberg. Koffka, K. (1931). Die Wahrnehmung von Bewegung [The perception of motion]. In A. Bethe, G. V. Bergmann, G. Embden, & A. Ellinger (Eds.), Handbuch der normalen und pathologischen Physiologie: Receptionsorgane II [Manual of normal and pathological physiology: Sense organs II] (pp. 1166-1214). Beriin, Germany: Springer. Lasersohn, W. (1912). Kritik der hauptsächlichsten Theorien über den unmittelbaren Bewegungseindruck [Critique of the main theories of the immediate impression of motion]. In F. Schumann (Ed.), Untersuchungen über die Wahrnehmung der Bewegung durch das Auge [Studies on the perception of motion through the eyes]. Zeitschrift für Psychologie, 61, 81-121. Metzger, W. (1940). Friedrich Schumann. Ein Nachruf. [An obituary]. Zeitschrift für Psychologie, 148, 1-18.
Schumann, F. (1887). Electromagnetische Rotationserscheinungen flüssiger Leiter [Electromagnetic rotationphenomena of liquid conducting mediums]. Annalen der Physik und Chemie, XXXII, 141-165. Schumann, F. (1893). Über die Schätzung kleiner Zeitgrößen [On the estimation of small amounts of time]. Zeitschrift für Psychologie und Physiologie der Sinnesorgane, IV, 1-69. Schumann, F. (Ed.) (1907). Bericht über den IL Kongreß für experimentelle Psychologie in Würzburg vom 18. bis 21. April 1906 [Report on the Second Congress of Experimental Psychology in Wuerzburg (18th April - IV April 1906)]. Leipzig, Germany: Johann Ambrosius Barth. Schumann, F. (Ed.) (1912). Bericht über den V. Kongreßfür experimentelle Psychologie in Berlin vom 16. bis 20. April 1912 [Report on the Fifth Congress of Experimental Psychology in Berlin (16* April - 20* April 1912)]. Leipzig, Germany: Joharm Ambrosius Barth. Schumann, F., & Katz, D. (Eds.). (1933). Friedrich Schumann. Zum 70. Geburtstag am 16. Juni 1933 und zum 25 jährigen Redaktionsjubilaeum (März 1934) überreicht [On the occasion of Friedrich Schumann's 70th birthday and his 25th anniversary as editor]. Zeitschrift für Psychologie, 129, I-II.
Sommer, R. (1904/1984). Die Ausstellung von experimental-psychologischen Apparaten und Methoden bei dem Kongreß ßr experimentelle Psychologie Gießen 18.-21. April 1904 [Exhibition of experimental instruments and methods at the Congress of Experimental Psychology in Gießen (18* April 21" April 1904)]. Passau, Germany: Passavia. Spillmann, L. (Ed.). (2012). Max Wertheimer: On perceived motion and figurai organization. Cambridge, MA: MIT Press. Spindler, A., & Hoyer, J. A. (1908). Spindler & Hoyer. Apparate für psychologische Untersuchungen [Instruments for psychological studies]. Preisliste XXJ. Göttingen, Germany: Spindler & Hoyer. Wagner, J. (1918). Experimentelle Beiträge zur Psychologie des Lesens [Experimental contributions to the psychology of reading]. Leipzig, Germany: Johann Ambrosius Barth. Wertheimer, M. (1912). Experimentelle Studien über das Sehen von Bewegung [Experimental studies on seeing motion]. Zeitschrift für Psychologie und Physiologie der Sinnesorgane, 61, 161-265. Received July 5, 2013 Accepted October 1, 2013 •
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© 2014 American Psychological Association 1093-4510/14/$12.00 DOi: 10.IO37/a0O35332
SCHUMANN'S WHEEL TACHISTOSCOPE: Its Reconstruction and Its Operation Armin Stock University of Wuerzburg In the fall and winter of 1910, Max Wertheimer (1880-1943) performed his famous experiments on perceived motion, published in 1912. Besides slider experiments he mainly used a wheel tachistoscope developed by Friedrich Schumann (1863-1940) at the end of the 19th century. The Adolf-Wuerth-Center for the History of Psychology has several wheel tacbistoscopes in its collection of instruments. Their provenance can be traced back to the Institute of Psychology of the University of Frankfurt and the University of Zurich. It is very plausible that Wertheimer, who performed his experiments at the Frankfurt Institute, used one of them. But the wheel tachistoscope alone is not sufficient to reconstruct Wertheimer's original experiments. As always, the devil is in the details. Wertheimer's descriptions of the necessary accessories, a prism, a viewing device, and an electric motor to move the wheel, are rather sparse. This article describes the results of a search for traces in the literature, in archives, and in literary depositories to shed some light on Wertheimer's experimental equipment. As a result, it was possible to reconstruct the entire apparatus and to obtain the same optical impressions with the reconstructed devices as Wertheimer's observers reported. In addition, one of his results was replicated with new participants exactly 100 years after its first publication. Keywords: wheel tachistoscope, phi phenomenon, Friedrich Schumann, Max Wertheimer, apparent motion
The 61st volume of the Zeitschrift für Psychologie {Journal of Psychology) edited by Friedrich Schumann in 1912 consists of a total of six papers. Two of them deal with aspects of perceived motion and were written at the Institute of Psychology of the Frankfurt Academy directed by Schumann, which is the forerunner of the University of Frankfurt. For the first—and rather unknown—paper by Woldemar Lasersohn, entitled "Critique of the Main Theories of the Immediate Impression of Motion," Schumann was the instigator. About the second, a paper that became internationally
I am very grateful to Michael Wertheimer for many suggestions for improvement.s to an earlier version of this article. In addition, I thank the Adolf Wuerth GmbH & Co. KG for supporting the Adolf-Wuerth-Center for the History of Psychology. Correspondence concerning this article should be addressed to Armin Stock, Adolf-Wuerth-Center for the History of Psychology, University of Wuerzburg, Pleicherwall 1, D-97070 Würzburg, Germany. E-mail; Armin. [email protected]
recognized, titled "Experimental Studies on Seeing Motion" and written by private lecturer Dr. Max Wertheimer, Schumann said only: ". . . a fairly independent study about the same topic . . . " (Schumann in Lasersohn, 1912, p. 81). Schumann described it the same way at the Fifth Congress for Experimental Psychology in Berlin on the April 18, 1912 (Schumann, 1912, p. 181). Schumann, however, also wanted to study this issue himself and had already instructed a Mr. Chaym to conduct preliminary studies at the Psychological Institute in Berlin, many years before Wertheimer's research (cf. Lasersohn, 1912, p. 102). Therefore, a suitable experimental procedure had already been prepared and passed on to Wertheimer some years later, who then also became interested in questions related to the perception of motion (cf. Lasersohn, p. 81). The aim of our project was to reconstruct the apparatus and to use Schumann's experimental approach that he recommended to Wertheimer. A key component of this experimental approach is the wheel tachis149
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toscope developed by Schumann. Hence, it became necessary to go back into the 19th century and determine when Schumann constructed such an instrument and how it worked. Friedrich Schumann: His Life and the Development of His Wheel Tachistoscope Friedrich Schumann (Figure 1) was bom in Hildesheim near Hannover on June 16, 1863. In school he demonstrated a special talent for mathematics, as did Max Wertheimer (cf. Metzger, 1940, p. 2). This may have been one reason for Schumann's decision to study phys-
Figure I.
ics at the University of Goettingen. He began his studies in 1881 and finished them in 1885 with a mathematics-based thesis on "Electromagnetic rotation-phenomena of liquid conducting mediums" (Schumann, 1887). After that, he focused on psychological research and became an assistant to Georg Elias Mueller (1850-1934) in Goettingen. In 1892, he finished his habilitation work "On the estimation of small amounts of time." This paper (1893) developed a field of research important for the later construction of the wheel tachistoscope. From 1894 to 1905, Schumann was the first assistant to Carl Stumpf (1848-1936) in Berlin.
Friedrich Schumann (1863-1940) (cf. Schumann & Katz, 1933, p. I).
SCHUMANN'S WHEEL TACHISTOSCOPE
Late in this time period he made the acquaintance of the young student Max Wertheimer. In 1905, he was called to a chair at the University of Zurich, which he abandoned 5 years later in favor of a professorship at the Psychological Institute of the Frankfurt Academy as successor to Karl Marbe (1869-1953). Among his first assistants were Wolfgang Koehler (1887-1967) and Kurt Koffka (1886-1941). In the same year (1910), Max Wertheimer (1880-1943) became one of his assistants as well. With these young researchers, Frankfurt developed into the key location for the beginning of Gestalt Psychology. Schumann served as head of the Frankfurt Institute until 1928. In 1929, Wertheimer became bis successor. Schumann died in Frankfurt in 1940. Schumann developed his wheel tachistoscope during his years as assistant to Carl Stumpf in Berlin. Wertheimer used it for his studies of apparent motion, the so-called phi phenomenon. It is unclear exactly when Schumann began the construction of the wheel tachistoscope. The first hint of an existing functional instrument of this kind can be found in 1899, when officers of the psychological society in Berlin, its chairman Flatau and its secretary Giering, visited the psychological institute of Professor Stumpf. The tour through the Berlin Institute was among others conducted by private lecturer Dr. Schumann, who took the opportunity to introduce his visitors to his newly developed wheel tachistoscope. This instrument allows " . . . making objects visible within the field of vision of a telescope for a short period of time. It makes it possible to erase the afterimage after a variable and precisely measurable amount of time through intense illumination of the field of vision" (Flatau & Giering, 1899, p. 97). Furthermore, the instrument makes it possible for ". . . two objects to become visible after each other at any interval" (p. 98). Hence, it can control all conditions needed to produce apparent motion. A detailed description of the wheel tachistoscope exists in a report on an exhibit of experimental psycbological instruments and methods at the first Congress for Experimental Psychology in Gießen in 1904 (Sommer, 1904, p. 14). The tachistoscope with electric motor, by Schumann (Berlin), was exhibit number two. The following is an excerpt from its description;
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A wheel of approx. [3/4] m diameter is carried on an axis running on bearings . . . . The periphery of the wheel is a 10 cm wide metal strip which can be divided into 8 parts. One of these octants carries a gap of variable width and a neighboring octant an adjustable mirror, inclined 45° from the plane of the wheel. A telescope is attached to the instrument such that its field of vision is masked by the iron ring and only uncovered for a moment when the gap at the objecüve passes by . . . . The instrument is set in motion via the pulley of a small electric motor. Its rotation speed is held constant by a von Helmholtz centrifugal regulator. (Sommer, 1904, p. 14f.)
A footnote to the description refers to the producer of the tachistoscope, the Spindler & Hoyer Company in Goettingen. Figure 2 was printed in the company's catalog for 1908 (Spindler & Hoyer, 1908, p. 168). For the use of this tachistoscope, the company offered to supply a reading telescope (Figure 3) with a close focus area of less than 1 m (Spindler & Hoyer, 1908, p. 170). Schumann spoke twice about his wheel tachistoscope at the second Congress for Experimental Psychology in Wuerzburg in 1906. In addition to a description of the advantages of rotating tachistoscopes against falling ones (Schumann, 1907, p. 164), he indicated that he had improved his wheel tachistoscope for rapidly succeeding exposures of different objects; tbe new model was produced by tbe Zulauf & Co Company in Zurich (Schumann, 1907, p. 165). Additional evidence that a new and an old model of Schumann's tachistoscope existed is Koffka's (1931, p. 1170) statement that the "old model" of Spindler & Hoyer is pictured in Klemm's (1925, p. 93) work. A more important footnote stated that Schumann had already conducted tachistoscopic apparent movement experiments. He called the content of awareness, the perceived motion which appeared, in accordance with von Ehrenfels "Gestalt quality" (Schumann, 1907, p. 218). Wertheimer's Use of Schumann's Wheel Tachistoscope The well-known picture of Max Wertheimer from 1913 (cf. King & Wertheimer, 2005; Spillmann, 2012), where he is next to a wheel tachistoscope (Figure 4), shows that Wertheimer did not work with the tachistoscope produced by Spindler and Hoyer, but with the newer Zurich model. The Zurich model is different from the old model in its spoke structure and in
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m Figure 2. Drawing of the Schumann wheel tachistoscope in the catalog of the Spindler & Hoyer Company, Goettingen (1908. p. 168). The octants are separated by eight spokes.
its large cast-iron fly wheel. The structure of the outer ring, which is not divided into octants as in the old model, is also clearly different. Instead, there are viewing windows that can be opened and closed via shutters. The instrument Wertheimer used could not have been built in Frankfurt because of time constraints: Schumann was called to Frankfurt for the 1910 summer term, and Wertheimer already conducted his experiments during the autumn and winter of 1910 (cf. Wertheimer, 1912, p. 175). Hence, Schumann probably brought the instrument with him from Zurich to Frankfurt. In addition to the tachistoscope, Wertheimer needed a viewing scope for his experiments, as well as a prism and an electric motor. He named all these items briefly, but did not describe them in detail: The Schumann apparatus . . . has a prism close to the tachistoscopic wheel, beyond the distal lens of the telescope one looks through. This prism blocks the lower half of the lens, so that rays come into the upper half of the lens directly, while entering the lower half from the side. One exposure slit on the wheel exposes the upper half, whereas a second slit exposes the lower half. If the distance between the lens and the prism is small, then the whole circular surface of each momentary exposure field is seen at each of the two exposures. . . . The distance of the exposure fields from the prism
was about 80 cm. (Wertheimer, 1912, pp. 175. 177; Translation from Spillmann, 2012, p. 13f.)
A detailed examination of the 1913 picture gives the impression that Wertheimer is holding something in his hands. At first glance it seems to be a cigarette between the thumb and the forefinger of his right hand. But there are several reasons against this first assumption. The picture appears to be composed with the intent of showing Wertheimer with his apparatuses. In addition, a cigarette would most likely not have been accepted by the photographer, and we cannot detect any smoke in the picture. Finally, Wertheimer would have burned his left forefinger keeping a cigarette in this position. Another consideration must be kept in mind as well: The object seems to be almost transparent and because it is physically not possible to bend only two of the three joints of the left forefinger without pressing the forefinger against an object, Wertheimer was not holding it in his right but in his left hand. The straight finger position of the first two joints of Wertheimer's left forefinger gives us a hint about the length of the object: The distance between the fingertip and the second joint of a male adult's index finger tends to be between 4 and 5 cm in length.
SCHUMANN'S WHEEL TACHISTOSCOPE
t53
Figure 3. Drawing of the reading telescope available together with the wheel tachistoscope in the catalogue of the Spindler & Hoyer Company, Goettingen (1908, p. 170).
Hence, we can speculate that the object has a similar length. Finally two more physical observations give us further information about the nature of the object; Because there are no hard shadows, we can conclude that the photographer did not use a flash light. Instead he could have used two lamps: One lamp on the left side produced a small shadow of the stand on the cast iron wheel of the tachistoscope. The other lamp was opposite to Wertheimer's head to illuminate his face. This second lamp brightened up the stimuli on the table, as well as caused a slight reflection of the object in Wertheimer's hands. Finally, we can see a duplication of Wertheimer's left thumb, which is partly below the object. All these considerations lead to the conclusion that the object in Wertheimer's hand was the prism that he required for his experiments, and that it was about 4-5 cm long. Today's prisms often have an enhanced optic quality. However, Wertheimer could not have
used this technical improvement because it was not available before the 1920s. This information comes from the Qioptiq Company, today's successor company to the Spindler & Hoyer Company. It is still quite probable, however, that Wertheimer used a high-quality prism because it is virtually impossible to get a good reflective projection of a collaterally positioned object with a low-quality one. Wertheimer does not present any details about the viewing scope he used. Because of a distance of only 80 cm between the exposure fields and the prism, it can be concluded that a telescope with a low near focal point was needed. This was quite rare at the beginning of the 20th century. Wertheimer may have used the scope sold with the old tachistoscope model produced by Spindler and Hoyer (Figure 3). This scope was capable of producing a sharp image of objects at a distance of less than one meter. Another hint occurs in a dissertation by Julius Wagner (1918). He conducted studies on
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Figure 4. Max Wertheimer next to Schumann's taehistoscope (depicted from the back). Photo taken 1913 (cf. King & Wertheimer, 2005; Spillmann, 2012).
the psychology of reading at Schumann's Institute, using the wheel tachistoscope. Wagner wrote that the institute had two scopes available: a "Zeiss Triëder telescope" with a magnification of 2.5, as well as a smaller astronomic telescope (cf. Wagner, 1918, p. 11). Wagner attached the scope to the middle of the stand of the wheel tachistoscope via a tap hole. However, an inquiry to the archive of the Zeiss Company in Jena made clear that Zeiss did not distribute this kind of telescope in 1918 (e-mail message, Carl Zeiss archive, March 8, 2012). Triëder telescopes were only produced by the Goerz Company in Berlin, which later became a part of the Zeiss Ikon AG, but this company consolidation had not yet happened in 1918. A document corroborating the information of the
Zeiss archive exists among psychologist Friedrich Sander's (1889-1971) papers in our Center. Sander was professor of psychology at the University of Giessen between 1929 and 1933, and the photo in Figure 5 was taken there. The photo shows a Schumann wheel tachistoscope together with an electric motor and a Triëder telescope from the Goerz Company attached on a stand on the right side. The AdolfWuerth-Center owns the electric motor, controlled via cone pulleys and a rheostat, as well as the Triëder telescope. Thus, together with the wheel tachistoscope that came from Frankfurt, it became possible to reconstruct the complete apparatus used by Wertheimer (Figure 6). The last required components to repeat Wertheimer's studies were the objects to be
SCHUMANN'S WHEEL TACHISTOSCOPE
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Figure 5, Photo of a wheel tachistoscope. The picture was taken at the Institute of Psychology of the University of Gießen between 1929 and 1933. In addition to the wheel tachistoscope, the electric motor as well as the Triëder telescope from the Goerz Company can be recognized. (Collection AWZ Würzburg)
presented. Wertheimer proposed that they should be presented on black exposure fields in form of white or colored stripes (cf. Wertheimer, 1912, p. 176). Because the original objects were not in our collection, our restoration workshop produced appropriate replicas. Following Wertbeimer's description we used white stripes of 1 X 6 cm length and illuminated them laterally with a small photo lamp (cf. Wertheimer, 1912, p. 177). Replication of Wertheimer's Apparatus and Procedure The reconstruction of the historic apparatus and procedure went as follows: First, the wheel tachistoscope was placed on a sufficiently wide and long table. For safety reasons, it was securely attached with two bar clamps. Tbe electric motor was placed to the left of the tachistoscope and was connected to the flywheel of the tachistoscope via a belt. Unfortunately, it was not possible to use tbe historic electric motor mentioned above because the risk of damaging it was too high. Instead, we used a new three-phase induction motor that can be
accurately adjusted. In front of the lower middle part of the outer crest of the tachistoscope, a viewing scope was adjusted toward both windows of the wheel flange, such that the stimulus 80 cm behind the tachistoscope could be seen through the upper window, and the stimulus placed at a distance of 80 cm to the right of the tachistoscope could be seen through the lower window. To make the latter object visible through the lower half of the scope, a reflecting 90°-prism with an edge length of 50 X 50 mm from the Qioptiq Company was placed directly opposite the scope on an adjustable stand, as close as possible behind the tachistoscope's outer wheel flange. A certain amount of practice was needed to adjust all elements of the apparatus so as to produce an optimal impression of motion. In addition, this adjustment procedure had to be conducted anew for every test person, and under certain circumstances a new adjustment was also required within a given test series. The Goertz Triëder scope in our collection would have been suitable for the replication of Wertheimer's experiments despite its age. But we did not want to take the risk of damaging it
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Figure 6. Reconstruction of the apparatus shown in Figure 5. This reconstruction was achieved by using items in the collection of the Adolf-Wuerth-Center.
and therefore used a new macroscope from the Minox Company (MS 8 X 25). It was attached directly to the tachistoscope's stand. But the rotation of the tachistoscope led to slight vibrations at higher speeds, which distorted the resulting image impression. Therefore, we decoupled the scope and the tachistoscope and placed the scope at the same distance on a tripod in front of the tachistoscope. Calculation of Exposure Times The exposure times of the two stimuli were determined by the speed of the wheel tachistoscope rotation and by the opening of the exposure fields in degrees. Wertheimer used a stop watch for measuring the time needed for 20 wheel rotations. For example this could have taken 20.4 s. This time divided by 20 yields 1,020 ms for a single 360° rotation of tbe wheel. If the opening of the exposure fields is for example, 7°, the exposure time is (1020/360) X 7 = 19.83 ms per exposure field (cf. Wertheimer, 1912, p. 178). In our experiments, we determined the rotation speed of the wheel tachistoscope with a modern laser analyzer in revolutions per minute (rpm) and calculated the exposure times correspondingly.
Replication of Key Results of Wertheimer's 1912 Publication Wertheimer conducted his research with only three observers—a relatively small sample by today's standards. He wrote: "It turned out that a large number of observers was not necessary since the characteristic phenomena were altogether unequivocal, spontaneous and compelling" (Wertheimer, 1912, p. 177; Translation from Spillmann, 2012, p. 14). His observers were Dr. Wolfgang Koehler, Dr. Kurt Koffka, and Dr. Mira Klein-Koffka. His participants had an average age of 24 years because Kurt Koffka and Mira Klein-Koffka were both born in 1886 and Wolfgang Koehler in 1887. Wertheimer presented his results for the three distinctive perceived impressions on pages 179 and 180 in two tables. These three impressions are succession, optimal motion, and simultaneity. In one of his experiments, two white stripes on a black background with a common vertex at an angle of 45° were presented in the two exposure fields. The difference between Wertheimer's first and second table of results concerns only a variation in the size of the exposure fields. He concluded that not the size of the exposure fields but the time span of the interval t between the exposure fields is essen-
SCHUMANN'S WHEEL TACHISTOSCOPE
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Table 1 Results of the Replication of Wertheimer's Research After 100 Years With the Schumann Tachistoscope Wertheimer (1912, p. 179f)
Stock (2012)
Interval t(ms)
M
SD
n
M
SD
n
Succession Ideal motion Simultaneity
165,50 57,87 32,66
12,50 9,66 1,59
2 8 6
217,09 56,74 34,52
26,04 2,35 1,80
4 4 4
Note. Although Wertheimer only used the data of three participants, partly higher data can be found in the column "n." They indicate how many values of the three participants were used for the calculation of the arithmetic means.
tial for the impression of motion (Wertheimer, 1912, p. 179). We used openings of 7° and adjusted the interval t to 16° (Wertheimer, 1912, p. 178). Table 1 shows the results found for the three distinctive states of motion for both: Wertheimer's data and the data of our replication. The two sets of data are quite comparable, even though the average age of our four participants was (at nearly 34 years) higher than the average age of the participants in Wertheimer's original experiment. Apparently age is not an important variable in these studies. These results show a striking correspondence between the results of Wertheimer's experiments and our replication for the states of ideal motion and simultaneity. Perhaps for the succession state our observers might have used a stricter criterion, which may have led to the longer intervals t in our replication compared with Wertheimer's original results. However, our replication of Wertheimer's experiments of 100 years ago demonstrates that even today the same results can be produced using Schumann's original wheel tachistoscope. A video documenting the three states of movement has been prepared: It is available on the Internet pages of the Adolf-Wuerth-Center.' Summary and Discussion The project to reconstruct and use one of Schumann's historic tachistoscopes in our collection, which Wertheimer probably worked with in autumn/winter 1910, was successfully completed. One challenge was to find out what kind of scopes could work in the experiments with the wheel tachistoscope and what kind of reflecting prism was used by Wertheimer. Research in the literature, in archives, and in literary depositories yielded suggestions that experiments with wheel tachistoscopes were
carried out with the Triëder telescope manufactured by the Goertz Company or with a reading telescope offered by Spindler and Hoyer. We estimated the size of the reflecting prism from an object apparently held by Wertheimer in a well-known 1913 photograph (Figure 4). The length of the prism appears to be about the length of the first two joints of his left forefinger, which probably is about 4-5 cm. Hence, we used a prism with an edge length of 5 cm. It is unlikely that Wertheimer used a smaller prism because it would not have been possible to produce a good separation between the upper and the lower exposure fields with the window size of the tachistoscope with a smaller prism. A larger prism would not have been of any advantage for Wertheimer and would have been significantly more expensive. It turned out that using the wheel tachistoscope required a certain amount of practice for both experimenter and participant. Aside from such technical challenges as the adjustment of the viewing scope, the prism, and the stimuli and the regulation of the motor speed via cone pulleys and a rheostat, there were significant challenges for the participants who needed a good ability to observe and concentrate. They also needed to be able to direct their attention in particular ways when instructed to do so. Furthermore, the seating position when looking through the scope in direct proximity in front of a rotating disk was uncomfortable. Unfortunately, Wertheimer did not report whether he measured the distinctive impressions (succession, ideal motion, simultaneity) with increasing or decreasing speed of rotation. ' http://www.awz.uni-wuerzburg.de/archiv/film_foto_ tonarchiv/filmdokumente/max_wertheimer_und_das_ schumannsche_tachistoskop/tachistocope_in_action/
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Doubtless the use of a method of limits or adjustment could have increased the precision of measurement. Finally, it must be mentioned that a literature search revealed that Wertheimer carried out experiments that were prethought and prepared by Schumann. He then habilitated with his report on these experiments (cf. Gundlach's contribution in this issue). Wertheimer also did not use the tachistoscope distributed by Spindler and Hoyer, but an improved model built by the Swiss company Zulauf and Co following Schumann's guidelines. Nevertheless, Wertheimer earns credit for having drawn conclusions from his experiments that were meaningful for his time. His studies played a major role in the development and expansion of Gestalt psychology. References Flatau, T. S., & Giering, H. (1899). Psychologischer Verein zu Berlin. Sitzungsberichte [Psychological Society Berlin. Meeting reports]. Zeitschrift für Pädagogische Psychologie, I, 95-100. King, D. B., & Wertheimer, M. (2005). Max Wertheimer and gestalt theory. New Brunswick, NJ: Transaction Publishers. Klemm, O. (1925). Wahmehmungsanalyse [Perception analysis]. In E. Abderhalden (Ed.), Handbuch der biologischen Arbeitsmethoden [Manual of biological working methods] (pp. 1-106). Beriin, Germany: Urban & Schwarzenberg. Koffka, K. (1931). Die Wahrnehmung von Bewegung [The perception of motion]. In A. Bethe, G. V. Bergmann, G. Embden, & A. Ellinger (Eds.), Handbuch der normalen und pathologischen Physiologie: Receptionsorgane II [Manual of normal and pathological physiology: Sense organs II] (pp. 1166-1214). Beriin, Germany: Springer. Lasersohn, W. (1912). Kritik der hauptsächlichsten Theorien über den unmittelbaren Bewegungseindruck [Critique of the main theories of the immediate impression of motion]. In F. Schumann (Ed.), Untersuchungen über die Wahrnehmung der Bewegung durch das Auge [Studies on the perception of motion through the eyes]. Zeitschrift für Psychologie, 61, 81-121. Metzger, W. (1940). Friedrich Schumann. Ein Nachruf. [An obituary]. Zeitschrift für Psychologie, 148, 1-18.
Schumann, F. (1887). Electromagnetische Rotationserscheinungen flüssiger Leiter [Electromagnetic rotationphenomena of liquid conducting mediums]. Annalen der Physik und Chemie, XXXII, 141-165. Schumann, F. (1893). Über die Schätzung kleiner Zeitgrößen [On the estimation of small amounts of time]. Zeitschrift für Psychologie und Physiologie der Sinnesorgane, IV, 1-69. Schumann, F. (Ed.) (1907). Bericht über den IL Kongreß für experimentelle Psychologie in Würzburg vom 18. bis 21. April 1906 [Report on the Second Congress of Experimental Psychology in Wuerzburg (18th April - IV April 1906)]. Leipzig, Germany: Johann Ambrosius Barth. Schumann, F. (Ed.) (1912). Bericht über den V. Kongreßfür experimentelle Psychologie in Berlin vom 16. bis 20. April 1912 [Report on the Fifth Congress of Experimental Psychology in Berlin (16* April - 20* April 1912)]. Leipzig, Germany: Joharm Ambrosius Barth. Schumann, F., & Katz, D. (Eds.). (1933). Friedrich Schumann. Zum 70. Geburtstag am 16. Juni 1933 und zum 25 jährigen Redaktionsjubilaeum (März 1934) überreicht [On the occasion of Friedrich Schumann's 70th birthday and his 25th anniversary as editor]. Zeitschrift für Psychologie, 129, I-II.
Sommer, R. (1904/1984). Die Ausstellung von experimental-psychologischen Apparaten und Methoden bei dem Kongreß ßr experimentelle Psychologie Gießen 18.-21. April 1904 [Exhibition of experimental instruments and methods at the Congress of Experimental Psychology in Gießen (18* April 21" April 1904)]. Passau, Germany: Passavia. Spillmann, L. (Ed.). (2012). Max Wertheimer: On perceived motion and figurai organization. Cambridge, MA: MIT Press. Spindler, A., & Hoyer, J. A. (1908). Spindler & Hoyer. Apparate für psychologische Untersuchungen [Instruments for psychological studies]. Preisliste XXJ. Göttingen, Germany: Spindler & Hoyer. Wagner, J. (1918). Experimentelle Beiträge zur Psychologie des Lesens [Experimental contributions to the psychology of reading]. Leipzig, Germany: Johann Ambrosius Barth. Wertheimer, M. (1912). Experimentelle Studien über das Sehen von Bewegung [Experimental studies on seeing motion]. Zeitschrift für Psychologie und Physiologie der Sinnesorgane, 61, 161-265. Received July 5, 2013 Accepted October 1, 2013 •
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