J.— PSYCHOLOGY 199 



II. Experimental Objections to the Transmission Theory, 



Although the way was undoubtedly paved for criticism of the trans- 

 mission theory by earlier researches, the main attack on it developed from 

 Wertheimer's investigation (published in 1912) of the so-called ' phi- 

 movement ' which results from successive stimulation at certain intervals 

 of two retinal points. This is one of many known examples of the 

 appearance in perception of something which does not exist in the pattern 

 of stimulation. On the retina there is only intermittent stimulation of 

 stationary points ; in the perceived world there is movement. Wertheimer 

 argued that this movement must be regarded as a genuine element in the 

 total system of physiological events which determine the perception, and 

 that it is not to be explained as an illusion of judgment. These experi- 

 ments formed also the starting point from which Kohler launched his 

 more general attack on the ' constancy hypothesis ' (i.e. the transmission 

 theory of perception). 



Criticism of the transmission theory and exploration of the implications 

 of the inclusion of the brain as a factor in vision might obviously have 

 started from other investigations. It is not my purpose to discuss 

 Wertheimer's work or the arguments that have taken place about it, but 

 instead to invite you to consider these problems in connection with a 

 field of experimental study with which I happen to be more familiar. 



We may begin by considering a simple experiment which can be 

 performed by anyone with no more complicated apparatus than an oval 

 table-mat or a sheet of cardboard and a pair of scissors. 



We place on a table an elliptical object with its long axis pointing 

 directly to and from the observer. If his head is directly above the object, 

 it will, of course, look elliptical. If now he moves his head from the 

 position directly above, but still keeping it in the vertical plane passing 

 through the long axis, the object will at first still look elliptical, but with 

 a smaller apparent elongation than when it is viewed from directly above. 

 If the head is now lowered, but still kept in the same plane, the apparent 

 shape of the object becomes nearer and nearer to a circle. It then becomes 

 truly circular and, if the head is still further lowered, the object appears 

 elliptical again only now with the really longer axis apparently the shorter. 



So far everything appears to be as one would predict on the transmission 

 theory by the elementary principles of perspective. Measurement of the 

 actual angles at which these various appearances are found reveals, how- 

 ever, a considerable discrepancy from the expectations aroused by the 

 transmission theory. At the height, for example, at which the ellipse 

 looks circular, it is found that the retinal image is not of a circle but of 

 an ellipse with the vertical axis much shorter than the horizontal, that is, 

 an ellipse flattened in the opposite direction. It is as if the shape that is 

 seen (the phenomenal shape) is in between the real physical shape of the 

 ellipse and the shape that is projected on the retina (which we may call 

 the stimulus shape). 



The natural expectation on the transmission theory would be that the 

 stimulus shape and the phenomenal shape would be identical. Plainly 

 they are not, and the discrepancy is large enough to show clearly without 

 any great refinement of measurement. Can we save the transmission theory 



