PHENOMENA OF LIGHT AND COLOUR. 67 



boundary of the Diaphanous which is in it is something real. 

 The phenomena show clearly, he says, that this is colour, 

 for colour either is in the boundary or is the boundary, 

 wherefore the Pythagoreans considered the external surface of 

 a body to be the same as its colour. Aristotle proceeds to say 

 that colour is not the boundary of the body itself, but is 

 in the boundary, and that the nature or constitution of the 

 inner parts of the body is the same as that which, at the 

 surface of the body, constitutes colour.* Again, he says 

 that colour is continuous with light,! and, as has been 

 stated already, he considers light to be the colour of the 

 Diaphanous. 



It will be noticed that the above statements are of the 

 nature of definitions which give very little assistance in deter 

 mining how colour effects are produced. The Diaphanous, 

 on which all Aristotle s conceptions about colour seem to 

 depend, was a mental conception, or, if intended to be 

 something concrete, its nature is difficult to understand. The 

 boundary of the Diaphanous in a body is, however, treated 

 by him as if it were something real, in which the colour of 

 the body existed. According to such views, the green colour 

 of an emerald or the yellow colour of an ingot of gold is 

 manifested only by the external surface of the Diaphanous 

 in the emerald or ingot, but the same colour would be 

 manifested by any other section of the Diaphanous in these 

 bodies if, by breaking the emerald or cutting the ingot, such 

 section coincided with the plane of breaking or cutting. 

 However difficult it is to understand some of his statements 

 about colour, it seems to be quite clear that he considered it 

 to be a boundary phenomenon. 



Both air and water, he says, have a colour of some kind, 

 but, inasmuch as air and water have no definite or fixed 

 boundaries, their colours vary according to the distance from 

 which they are seen. The colours of solid bodies, on the 

 other hand, remain the same, unless the action of anything 

 surrounding or near them causes a change.! The last 

 clause of this passage is one of the few assertions to be 

 found in Aristotle s works which suggest that he considered 

 the colour of a body to depend on anything but the nature 

 of the Diaphanous. There seems to be nothing to anticipate, 

 however, the modern view that the colours of bodies are not 



* De Sensu, &amp;lt;c., iii. 439a. f Physics, vii. c. 2, s, 4. 



I De Sensu, dc,, iii. 439&. 



