382 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



are compounded of four ; white, black, bright, and red. Bright 

 when mixed with red and white becomes golden-yellow ; red 

 blended with black and white yields violet. 



From statements in the " Timaeus " and " Republic " it would 

 seem that Plato, unlike Democritus, believed in the objective 

 existence of colour in things ; but, as Helmholtz remarks ( 5 ), his 

 views seem to have varied. In the " Thesetetus," colour is con- 

 sidered from an entirely different standpoint, as will be clear from 

 the next quotation : 



" We shall see that every colour, white, black, and every other 

 colour, arises out of the eye meeting the appropriate motion, and 

 that what we term the substance of each colour is neither the 

 active nor the passive element, but something which passes between 

 them and is peculiar to each percipient. . . ." 



" When the eye and the appropriate object meet together 

 and give birth to whiteness and the sensation of whiteness which 

 could not have been given by either of them going to any other 

 object ; while the sight is flowing from the eye, and whiteness 

 from the colour-producing element, the eye becomes fulfilled with 

 sight and sees, and becomes not sight but a seeing eye ; the object 

 which combines in forming the colour is fulfilled with whiteness 

 and becomes not whiteness but white " ( 6 ). 



Aristotle's (B.C. 384-322) theory of vision is of very great 

 importance in the further development of the subject, and indeed 

 still survives in a modified form. We must therefore examine it 

 more closely. 



According to Aristotle, the object of sight is colour. Colour 

 is at the surface of all visible objects, but, in order to be seen, 

 requires the presence of light, which is the medium of vision. 1 



Light again pre-supposes a diaphanous substrate which in its 

 turn is the medium of light. Examples of this " diaphanous " are 

 air, water, and many solids. The realisation or actualisation of 

 this potential quality of being diaphanous is light, its absence 

 darkness. When the former condition of actual light is estab- 

 lished in the diaphanous medium, any coloured body sets up a 



1 Cf. Lucretius, De Her. Nat., bk. ii., 795 



' ' Praeteria quoniam nequent sine luce colores 

 Esse, neque in lucem existunt primordia rerum, 

 Scire licet quam sint nullo velata colore." 



The whole passage, from 730 to 833, is of much interest in this connection. 



