514 



NATURE 



[March 31, 1892 



First of all, I must draw attention to a very important 

 distinction between colours and colours. It is, of course, 

 clear that a colour may be due either to a pigment or to 

 the physical structure of the coloured body ; and it was 

 therefore very necessary for me to find out, so far as 

 possible, which of the colours I might have to deal with 

 were physical, and which pigmental. With regard to some 

 of these, it could tolerably safely be conjectured — merely 

 from the appearance— that they were simple physical 

 colours ; in such cases, I mean, more especially, where 

 there was a distinct sheen or glow in the colour ; and I 

 have been able to confirm various conjectures that had 

 previously been made, both by others and myself, as to 

 these physical colours. But in many other cases — indeed, 

 in the majority — nothing but experiment could decide 

 the question ; and in some instances the decision has 

 been as unexpected as disappointing to me. In order 

 now to classify the results that I had obtained, and to in- 

 troduce as much order and method as possible into my 

 explanations of them, I have already ventured to propose ^ 

 the following scheme of colours: (i) pigmental colours ; 

 (2) interference colours, which include a very large num- 

 ber of insect examples, besides, of course; the iridescent 

 colours displayed by the wings of dragon-flies, May-flies, 

 &c. ; (3) reflection colours, other than the interference 

 colours — these will be found to include all the white- 

 winged species that I have examined ; and (4) it seems 

 necessary to have a class of simple absoi-ption colours, in 

 order to include all those cases of black in which no 

 pigment can be found, but, apparently, all the light-rays 

 are absorbed in a dense coating of scales. 



The limits of space at my disposal compel me to pass 

 over the colours black and white with the remark that as 

 to the former, with one or two dubious exceptions, it can 

 be affected by no reagents, and I have, therefore, con- 

 cluded it to be not pigmental, but simply a " physical " 

 absorption colour ; full details as to this will be found in 

 the Entomologist. As to white, I have similarly failed to 

 find any pigment, or to obtain any reaction, except with 

 Melanargia galathea, and two or three white-fringed 

 species ; in these instances the white is changed to a 

 deep yellozv, which presently dissolves in the reagent. 

 The explanation of this I must defer until the pheno- 

 mena of yellow have been discussed. For the rest, white 

 is evidently simply a reflective colour, and not pigmental. 



We now have to consider in succession the five colours 

 blue, green, red (and pink), yellow (and orange), and 

 chestnut ; and, first of all, I must recur to what was said 

 above on the criteria of physical and pigmental colours. 

 Referring my readers to the condensed tables of results, 1 

 given at the end of this article, I think — as the results of I 

 what I have been able to learn from my experiments — 

 that the following rules may be laid down. There are j 

 certainly two ways in which a pigment colour may be I 

 affected, and either effect is conclusive evidence of the 1 

 presence of a pigment. Firstly, the colour may be dis- 

 solved out ; the liquid is left more or less deeply coloured, 

 and the wing is white, or colourless.^ This is the case 

 i^oide^ tables) with all the yellows and chestnuts that are 

 sensitive at all to the reagents, and also with the pigment 

 greens in most instances. It is very important to observe 

 that this change from a yellow or chestnut wing to a 

 white one does not imply any change from a yellow pig- 

 ment to a white one — as might at first be supposed from 

 merely glancing at the records in the tables : it is not so. 

 The change is due simply to a solution of the pigment, 

 •which has originally been developed, not from a white 

 pigment, but in a white, i.e. previously unpigmented, 

 wing. It will be necessary to refer to this again later in 

 discussing the behaviour of ^. ^^ii/a//^^^. It is scarcely 

 necessary to point out how important a bearing the inter- 



I Entomologist, September 1890. 

 ^ The apparent exceptions of Vanessa 

 plained under " chestnut," infra. 



and V. atalanta will be ex- 



pretation of such results has upon our view of the nature 

 of white. 



To proceed : the second criterion for pigment colour 

 (and this, it is needless to say, cannot concur with the 

 former) is What I have denominated the " reversible " or 

 " reversion " effect ; and this I have found only in the 

 case of red,^ which, I may observe, is out and away the 

 most satisfactory colour to experiment upon. In these 

 cases, the effect of the reagents (but chiefly of the 

 acids) is to convert the red colour into a fine yellow 

 or orange, from which the original red can be com- 

 pletely recovered by appropriate means, as will be ex- 

 plained in due course : here, again, there is indubitably 

 a pigment in evidence. In some cases, however, where 

 there is neither solution nor any "reversion" effects, but 

 yet a (sudden) change from the original colour, it is ex- 

 tremely perplexing to decide whether we have to do with 

 a pigmental or with a physical colour. Instances of this 

 will be found in the tables, among the greens {e.g. 

 Argynnis and TJiecla) and the blues {e.g. the Lyccenidce). 

 In such cases I have not ventured to pronounce defi- 

 nitely in favour of either view, although it appears to 

 me that the evidence is strongly in favour of such colours 

 being simply physical."-^ I do not think that there is the 

 least difficulty, theoretically, in supposing such reactions 

 to take place with mere physical colours ; since the wing- 

 surface, when soaked — even by an indifferent or neutral 

 fluid — might well be so affected, at least temporarily, 

 as to alter its reaction in the light rays, i.e. to alter the 

 resulting colour. In such cases, then, we have an element 

 of doubt to contend with. 



Then, as to undoubtedly physical colours, there are 

 certain blues and greens which, when examined with the 

 naked eye even, can be seen to be, not a continuous 

 patch of colour, but a mass of— so to speak— distinct 

 dots.' Speaking now on the strength of my experience 

 with such, I think I am justified in stating that these 

 may safely be pronounced oft"-hand, without experiment, 

 to be physical. When such colours are tested with the 

 reagents, they may either be entirely unaffected, or the 

 colour may disappear, but reappear (usually quickly) on 

 drying. It may prima facie be retorted, and not un- 

 reasonably, that these should be considered pigment 

 colours showing the reversion effect ; but — as will be seen 

 after the reversion effect of red has been described — 

 there is really no similarity at all ; and there can scarcely 

 be a doubt that these are merely physical colours. 



Again, a brilliant metallic-looking colour may be 

 changed to a different colour, or sometimes to a dead 

 brown or blackish {vide tables : green), and this effect 

 may be either temporary or permanent ; and yet, from 

 the general appearance of the colour before and after the 

 experiment, one may feel thoroughly assured that it is 

 only a physical colour.^ And lastly, in such cases, a 

 brilliant blue, e.g., may be unaffected by most reagents 

 (or only temporarily so), whilst such a reagent as nitric 

 acid or potassic hydrate may permanently dull or destroy 

 the colour. This is perfectly intelligible, since in such 

 cases the powerful reagent has no doubt damaged the 

 surface structure. I have thought it only right and fair 

 thus to outline the data on which my conclusions con- 

 cerning the nature of these colours have been founded, 



' One or two instances have recently been noticed of partial reversion of 

 a colour originally reddish-brown among the Uoiubyces. These seem to be 

 connecting-links between yellow and chestnut descended colours (see later). 



^ Facts in support of this view will be quoted in their proper place. But 

 I may be allowed to say that one's judgment in such cases mw^lhs. partially 

 founded on observation of appearances and conditions that in their nature 

 do not admit of being described or formulated, but appeal to an observer 

 who has learnt by experience to interpret such indications. It will there- 

 fore be understood that, throughout this article, the actual evidence for my 

 conclusions is apt to be somewhat discounted when the attempt is made to 

 briefly convey it in words. 



3 I ought to add that it is not always safe to assume, merely because the 

 reagent has become coloured, that the surface colour under e.\amination is a 

 pigment colour ; for some recent observations have led me to believe that 

 there may be an unapparent pigment present in wings whose surface is 

 physically coloured only. 



NO. 



I I 70, VOL. 45 J 



