116 THE ENTOMOLOGIST. 



down the rule that when a patch of colour visibly consists not of 

 a continuous mass, but of a number of isolated distinct spots of 

 colour, that colour is certainly physical. So far as my experience 

 goes, this kind of appearance is found only in the metallic greens* 

 and the blues; among the latter Papilio machaon affords an 

 excellent illustration. I hope that my description of the kind of 

 effect to which I wish to draw attention is intelligible ; but if not, 

 I will ask any of my readers to carefully examine the blue 

 markings on P. machaon, after doing which I think that they 

 will no longer be under any doubt. 



Now, as to the reactions of such " physical " blues and greens, 

 it must not be hastily assumed that because such colours are not 

 pigmental, therefore they will be totally unaffected by any 

 reagents. This would be erroneous. Of course there can be no 

 dissolution effect, neither can there be any " reversible " effect, 

 such as the reds display. But it must be remembered that, 

 since the colour in such cases depends on the molecular structure 

 of the surface, any temporary or permanent alteration of this 

 surface may be expected to temporarily or permanently alter the 

 colour. It would be strange indeed, if a thorough soaking with 

 even any indifferent liquid did not temporarily affect the molecular 

 structure, and so alter its effect on the light rays ; much more 

 might we anticipate that any such powerful and destructive 

 reagents as caustic potash, or nitric acid, would permanently 

 alter the surface structure, and so lead to a coloric alteration. 

 And this is just what we do find in many cases, as will more 

 clearly appear after the phenomena of blue species have been 

 discussed. Accordingly, one of three things may happen when 

 we experiment on these "physical" colours: — (1) They may be 

 unaffected; or (2) the colour may disappear, but gradually return 

 on drying ;t or (3) the colour may be permanently either altered 

 to, usually, another metallic-looking colour, or permanently 

 dulled or destroyed. 



Returning now to the three green species just referred to 

 (U. fulgens, P. gambrisius, and the Hesperia), we find that the 

 greens are changed to some sort of a bronze — a reaction which 

 is perfectly intelligible in the case of a physical colour. Since 

 further examples of such changes will be found among the blues, 

 we need linger no longer to consider them here; but I may 

 observe that the green of Parthenos gambrisius (upper surface) 

 suffered unusually for a colour of this claas, since the final effect 

 of most of the reagents was to destroy all lustre and colour 

 altogether, and I found no return after several days' standing. 

 The " physical " greens seems to possess by no means so 

 " strong a constitution " as the physical blues. 



* I ought, however, to add, that with some of these metallic colours it is a toss up 

 whether to call them green or yellow ; all depends on the angle at which the light 

 falls on them. 



t This is very marked in P. machaon. 



