STUDIES IN SPECIAL SENSE PHYSIOLOGY 381 



order relatively to one another ; (6) to their several shapes ; and 

 (c) to the position of each in its place. The subjective aspects 

 the qualities of sensible objects are all due to these three things. 

 Colour has no objective existence, since the colours of bodies are 

 due to the position of the atoms in them " ( 3 ). 



The views of Anaxagoras (B.C. 499-428) and Diogenes of 

 Apollonia (5th cent. B.C.) may be passed over rapidly. Anaxagoras 

 held, in opposition to his contemporaries, although the opposition 

 is more formal than real, that perception is the result not of like 

 operating upon like, but of the reaction between contrary and 

 contrary. The " image " is not reflected upon a part of like colour 

 to the object but upon a different colour. Diogenes, who be- 

 lieved that an all-pervading " air " was the ultimate agency in 

 nature, has left no distinct theory of colour vision. 



One would naturally expect that Plato (429-347), to whom we 

 now turn, would have powerfully contributed to the advancement 

 of our knowledge of the physiological psychology of vision, but 

 this is not the case. His account of the physical side of the 

 problem is contained in a passage in the " Tima3us," of which the 

 following quotation gives an idea : 



" And of the organs they first contrived the eyes to give light, 

 fixing them by a cause on this wise. They contrived that as much 

 of fire as would not have the power of burning, but would only 

 give a gentle light, the light of every-day life, should be formed 

 into a body ; and the pure fire which is within us and akin to 

 this they made to flow through the eyes in a single entire and 

 smooth substance, at the same time compressing the centre of the 

 eye so as to retain all the grosser element and only to allow this 

 to be sifted through pure. When therefore the light of day sur- 

 rounds the stream of vision, then like falls upon like, and there 

 is a union, and one body is formed by natural affinity according 

 to the direction of the eyes, wherever the light that falls from 

 within meets that which comes from an external body. And 

 everything being affected by likeness, whatever touches or is 

 touched by the stream of vision, their motions are diffused over 

 the whole body and reach the soul, producing that perception 

 which we call sight " (*). 



In the genesis of colour, particles are discharged from external 

 things and impinge upon the eye, some being larger, some smaller, 

 and some equal in magnitude to the parts of the eye. All colours 



