of some Brilliant Animal Colours. 103 



There is still another view, which indeed is that actually 

 maintained by Walter, whose argument and conclusion * it 

 may be well to quote : — 



" A further striking and at the same time more instructive 

 proof of the equivalence of the lustre of butterfly-scales and 

 the surface-colours of strongly absorbing dyes is to be found 

 in the changes which the colours of these organs exhibit 

 when immersed in fluids of varied refrangibility. These ex- 

 periments are instructive because they disclose the manner 

 in which the dye is contained in animal substances. 



" The experiments show that, except when it is deep blue 

 or violet, the lustre moves one or two colour-intervals in the 

 direction from the blue towards the red end of the spec- 

 trum with increasing refrangibility of the surrounding 

 medium, but at the same time becomes weaker. For example, 

 the scales of Morplio menelaus, L., which glitter green-blue in 

 air, become in ether (n — 1*36) a pure green, shining less 

 strongly, again in chloroform (?i = l*45) a yellowish green 

 and now decidedly weaker than in ether. In benzol (n=l'0'2) 

 and in bisulphide of carbon (?i = l*6I) the weak yellow-green 

 lustre is perceptible only with direct sunshine in a dark 

 room. In a similar manner the scales of Urania ripheus 

 shining green in air, in ether, alcohol or water become 

 golden yellow, the yellow red and the red blue, while in 

 benzol and bisulphide of carbon scarcely a trace of glitter 

 remains. 



" Where we know that the cause of the lustre is a dye, the 

 latter facts admit of but one interpretation — that in the case 

 of butterfly-scales we have to do with solutions of the dyes in 

 chitin, solutions whose refractivity for most of the spectrum 

 colours is nearly equal to those of benzol and bisulphide of 

 carbon, so that these colours, unless they are very strongly 

 absorbed by the solution, are practically not reflected in 

 their passage from the colourless liquids. Accordingly, the 

 dyes which give rise to lustre in the chitin-skin of insects, 

 and, ms we shall see presently, in the horny skin in birds, 

 are dissolved in the same fashion as cobalt oxide in blue 

 glass or organic dyes in a layer of solid gelatine, a conception 

 suggested in the simple observation of the scales by trans- 

 mitted light and confirmed by the facts above adduced." 



If Walter's argument and conclusion are accepted, the 

 difficulty, already considerable, of explaining the richness of 

 the animal colours is enhanced by the supposed dilution 

 of the dyes, and one can hardly fail to observe that a simpler 



* Loc. cit. p. 96. 



