﻿286 THE ENTOMOLOGIST. 



Thirdly, it seems to me that we ought to rank those colours 

 which are due to the partial or total absorption of light rays, 

 whereby either a complementary colour or a simple black is pro- 

 duced. It may well seem at first blush that this third class is 

 altogether erroneous and superfluous, since the definition of it is 

 the definition of the pigment class. For all that we mean by a 

 pigment — strictly — is a compound of such molecular structure 

 that it absorbs many or few, or all or none, of the light rays, 

 producing complementary colours, or black, or white. Neverthe- 

 less, I feel it necessary to constitute this third class in order to 

 accommodate colours that are apparently caused by absorption, 

 but cannot anyhow be chemically affected as one would expect 

 pigment colours to be. If asked to instance an morganic colour 

 of this class, I could not do better than point to a lump of coal 

 as a thoroughly characteristic example ; and no doubt various 

 other examples will occur to my readers. 



Of course I am fully aware how very arbitrary and artificial — 

 in final analysis — any distinction between chemical and "physical" 

 colours is. In the first place, all these sub-classes may be said 

 to overlap more or less : as already pointed out, reflected colours 

 and interference colours approach one another ; similarly reflected 

 and absorption colours also approximate in character* ; and, 

 lastly, all pigment colours are only a special case of absorption 

 colours. Moreover, ultimately, all colours are physical ; that is to 

 say, the only objective antecedent of the sensations that we call 

 colour is to be found in the inconceivably minute molecular 

 structure of matter, which so affects the ether vibrations as to 

 produce in us the sensations of colour. In this sense, therefore, 

 all colours — whether pure pigment colours, or interference 

 colours, or absorption colours — are physical. But, nevertheless, it 

 is eminently convenient to distinguish between, on the one hand, 

 chemical colours, due to pigments, which may be isolated and 

 utilised as such ; and, on the other hand, "physical" colours, in the 

 usual restricted and well understood use of the term. Always bear- 

 ing in mind, then, the caution just posited, we may freely speak of 

 chemical and physical colours, and need not trouble just now to 

 determine where, in absorption* colours, the physical ends and 

 the chemical begins. After all, this is no more perplexing or 

 arbitrary than the division between the overlapping sciences of 

 Physics and Chemistry, — a division that is purely arbitrary and 

 subjective, but very convenient for the purposes of human study ; 

 only Nature knows nothing of any such distinction. 



Let us, then, during the rest of this discussion, assume that 

 there are at least four classes of colours : — 



(a. Interference colours. 



1. Chemical or Pigment. 2. Physical. 



b. Reflected colours. 



c. Absorption colours. 



iSee preceding footnote. 



