198 SECTIONAL ADDRESSES 



and that a wholly different way of approaching the problems of visual 

 perception is necessary if we are not to be led astray. To say this is not 

 to deny the greatness of the achievements of those investigators in the 

 past whose work on vision was guided by this theory. Within a certain 

 limited field, it proved itself a fruitful guide to research. This field was 

 that of the sensory physiology of the retina. If we wish to discover what 

 is happening on the retina we must arrange conditions of experiment so 

 as to cut out, as far as possible, the complicating effects of the cerebral 

 components of the visual part of the nervous system. This was what 

 was done when the early experimenters made observations through tubes 

 or on black backgrounds. So such workers as Helmholtz, Konig, Abney 

 and a host of others made a firm foundation for a science of vision in the 

 sensory physiology of the retina. The error, however, has sometimes 

 been made of mistaking the foundations for the completed building. 

 When we get rid of tubes and black backgrounds and open both eyes to 

 look at objects surrounded by other objects we find that what we see 

 follows other and far more complicated principles than the laws of sensory 

 physiology. 



Against the successes of the transmission theory of vision in originating 

 fruitful lines of research, we must set its failures. Fruitful in the field of 

 sensory physiology, it left most of the field of perception a barren waste. 

 Its underlying assumption was that any visual experience which could be 

 exactly correlated with an event in the sense-organ was a true element of 

 experience (a ' sensation '), while those that could not were regarded as 

 due to the action of higher processes on these elements or were relegated 

 to the class of ' illusions ' or mistaken judgments. There were many 

 investigations of the perceptual experiences (such as that of depth) which 

 were attributed to correct judgments about sensations, although the 

 successes in this field were much less striking than in that of sensory 

 physiology ; the ' illusions,' however, were almost wholly abandoned to 

 anecdotal report. 



Fruitful experimental work on perception began when psychologists 

 began to doubt the validity of the distinction between true perceptions 

 and illusions. Katz, for example, in 191 1, devoted a considerable part 

 of a book to a description of such differences between colours as are not 

 due to differences between the local retinal stimulations. Thus the 

 difference between the appearance of yellow and of red is accompanied 

 by a difference in the bands of wave-lengths of light stimulating the eye, 

 and is regarded on any theory as a real psychological difference. There 

 is, however, also a difference between the appearance of a red book (a 

 surface colour) and a red patch seen in the spectrum of the same brightness 

 and the same composition of wave-lengths (a'film colour). This difference 

 is accompanied by no difference in the conditions of local retinal stimu- 

 lation and is, therefore, from the point of view of the transmission theory, 

 an illusory difference. It was only when psychologists adopted what has 

 been called the ' phenomenological ' point of view of regarding all differ- 

 ences in appearance as equally reputable objects of experimental study, 

 whether or not they were accompanied by differences in local physiological 

 stimulation, that the serious experimental study of such things as the 

 modes of appearance of colours became possible. 



