38 THE ENTOMOLOGIST S RECORD. 



I think the definitions, clearly explained in Mr. Tutt's notes, are as 

 good as any we can give in our present state of knowledge— always 

 premising the limitation of the word pigment, to that of a pigment- 

 factor. As Dr. Freer says, we see pigment granules in the scales, 

 under the microscope, but I fail to see that the instrument throws any 

 light on their nature, save as to structure. When we view them, 

 especially by transmitted light, all the conditions are altered, and we 

 get iridescence due to interference from one of its special causes — 

 diffraction of thin films ; but this is not the pigment we are attempting 

 to define. All tlie metallic colours of insects — imagines and pupje — 

 and their iridescence are of course due, as Dr. Freer says, to these latter 

 causes and not to pigment, but I very much doubt whether Dr. Freer 

 is right when he writes of " water changing the green of Thecla ruhi 

 to brown " as a refractive process. Does he mean that he considers it 

 an example of interference through thin films ? If so, surely it would 

 be iridescent ! I should call this change simply a chemical one, caused 

 by the direct action of the water on the pigment. 



I am afraid Dr. Freer will have to wait a long while for " colour to 

 be dissolved out and isolated." We may dissolve and isolate the 

 material that helps to give us the sense of colour — the pigment-factor 

 — but not the essential, unobserved vibrations of ether that strike the 

 nerve expansion of our eyes. This, Dr. Freer more or less admits in 

 his next paragi-aph (apparently contradicting the preceding), " even 

 my pigment granules have slender grounds to be called pigment, but 



how much less those colours whose claim rests on 



their naked eye appearance." Why less ? — W. S. Eiding. Nov. 3rd, 

 1894. 



With regard to Dr. Freer's last note, does he really mean what he 



says at the commencement of the note — " nor do those colours 



come under this head " ? Does this mean that these metallic colours 

 do not come under the head of transparent substances, or what ? or 

 should " come under this head " be deleted, as " nor " introduces the 

 sentence ? Is it a fact that the chitinous covering of the pupa is formed of 

 alternate layers of fluid and chitin, i.e., the structure -=clii tin, fluid, chitin, 

 fluid, and so on, just as an oyster shell equals a lamina of carbonate of lime, 

 then a lamina of membrane, carbo. of lime, membrane, &c., or does Dr. 

 Freer simply mean that the fluid contents of the pupa lie directly in 

 contact with the outer pellicle, which consists of a single laj'er? I be- 

 lieve the pellicle is transparent where metallic patches occur, and, if I 

 remember rightly, the emergence of tlie imagines from pupa} of Vanessa 

 urticae leaves the previous metallic areas quite transparent. It is some 

 years now since, in my essay on The genetic sequeyice of insect colours, 

 printed as the Introduction to British Noctuae, &c., vol. ii., I discussed 

 two kinds, or rather degrees, of black coloration one more or less pigmen- 

 ted by granules in the scales, the other absorbing light rays by means of 

 the peculiar structure of the cell wall. That this should be so, when one 

 observes the slow transition through highly pigmented purples and reds 

 to black, by every possible gi-adation, is to be expected, but I am much in 

 doubt whether the blacks of butterflies will, as a general rule, come under 

 one head more particularly than under the other. Instances of both kinds 

 will occur I doubt not, both in butterflies and moths. I do not doubt either 

 that Dr. Freer is quite correct, considering the modern view of science, 

 in stating that pigment is an expression of energy. It really has been 



