STUDIES IN SPECIAL SENSE PHYSIOLOGY 383 



movement in it, between object and eye ; this is the essential 

 process in colour perception. 



The diaphanous substrate, upon which depends the existence 

 of light and, a fortiori, colour, is not peculiar to the bodies called 

 transparent or diaphanous, but is a species of universally diffused 

 natural power ; it is not indeed capable of existence independently 

 of " body " but subsists in varying degrees in all bodies. The 

 colour of a body either forms its surface or is upon that surface, 

 the latter opinion being the more exact since the indeterminate 

 " diaphanous " of air and water exhibits colour, which, however, 

 owing to the indeterminate boundary, is variable. This explains 

 the changing hues of sea or sky. 



Bodies with a definite boundary have a fixed colour, so that 

 one might again define colour as the surface limit of the " dia- 

 phanous " in determinately bounded body. This definition is con- 

 sistent with the first given, viz. that which stimulates the actualised 

 " diaphanous " (light) between the object and the eye, but the 

 latter is a definition in terms of vision and the medium of vision, 

 the former in terms of the object as it exists apart from vision. 



Colour is a genus comprising seven species ; it is a quality and 

 cannot therefore exist without a substrate. The seven species are 

 white, black, golden-yellow, crimson, violet, leek-green, and deep 

 blue. The colour genus (like all other genera of sensible qualities) 

 consists of species lying between extremes ; outside these extremes 

 there can be no colours, between them are specific boundaries. 

 By subdividing the scale limited by the extremes, we cannot 

 obtain an infinite number of distinct colours because a sensible 

 quality is discrete not continuous. By dividing the substrate we 

 do not arrive at any new colour, the halves of a white object are 

 white. It is true that by sufficiently fine division no colour what- 

 ever may be perceptible, but on reuniting these portions we again 

 obtain white. The two limits are black and white ; when one is 

 actually existent the other is only potential. The transition from 

 white to black is effected through the successive degrees which are 

 the species of colour. The substratum, of which these are the 

 qualities, is one, and is in strictness that which is changed ; the 

 colours alternate. 



Colour is not purely subjective. It is true that it depends 

 upon the eye, but it also depends upon the object. Actual colour 

 depends upon the possibilities of these two being realised together, 



