﻿372 THE ENTOMOLOGIST. 



more fitly pointed out now than later, when any specific colours are 

 being discussed. The importance of this fact to a right under- 

 standing of the experiments can hardly be over-estimated ; but 

 unfortunately it is one to whose importance I was not alive until 

 I had been working some time. Reagents may affect pigment 

 colours in one of two ways : — they may dissolve out the pigment, 

 or they may simply alter it. The former action occurs with pro- 

 bably all chestnuts and yellows that are affected at all, and with 

 some greens ; the latter, with all reds without exception, with many 

 greens, and with some blues. The distinction is very important, 

 since, whilst dissolved colours can, obviously, never be restored, 

 those which are merely altered may, in a large number of cases 

 at any rate, be restored, by appropriate methods, to their original 

 appearance.* This will, I think, be acknowledged to be sufficiently 

 startling. It was a long time before I grasped these principles in 

 their full extent ; and then it not unnaturally appeared to me that 

 every colour change, when not due to dissolution, might be reversed, 

 and the original colour so reproduced. That this is the case, 

 owever, I find cannot be maintained, although (at least for the 

 present) I do not wish to absolutely deny its possibility. Neither, 

 on the other hand, would I wish to be understood as asserting 

 that in every "reversible" colour change no solution of pigment 

 takes place. On the contrary, I am inclined to think it at least 

 very possible that in most cases a slight solution may take place ; 

 at any rate I do not believe that the most " reversible " colour 

 could be altered and reversed, and so on, backwards and forwards, 

 an indefinite number of times ; certainly I believe that the colour 

 would gradually fade away in such a case. I very much regret 

 not being able to speak with more certainty on this matter, but, 

 as I have already said, it was only late in the day that Tctiscovered 

 these facts : had I known of them earlier — known what to observe 

 — it is possible that I might have found a solution effect produced 

 in cases where, as a matter of fact, I have not noticed any such. 



One characteristic difference I ought to add : — a solution 

 effect may be produced very rapidly in- some cases, but still the 

 time element is always present ; an alteration effect, whether 

 reversible or not, is in most instances (in all with the fittest 

 reagents) instantaneous. 



F. 



The two colours next to be considered — yellow and red — are 

 by far the most interesting of all, — that is, so far as concerns 

 these experiments : and they are so closely related, and a con- 

 sideration of the behaviour of either so indispensable to a proper 

 understanding of that of the other, that we shall do better to 

 examine them simultaneously rather than observe the formality 



* An extensive set of additional experiments on this question will be detailed in 

 its proper place. 



