SCIENTIFIC NOTES AND OBSERVATIONS. 189 



power which matter has, by reason of its form (striation, fine and 

 minute ribbing, ifec), of splitting up the light which falls upon it, 

 e.g. eggs of many lepidoptera, pearls, &c. If the colours of insects 

 fade and disappear, and it cannot be proved that the structure has 

 altered, then it is obvious that that colour cannot result from refrac- 

 tion or diffraction, but must l)e due to pigment — and vice versa. Study 

 the cases of insects hanging up outside tlie shops of some of our pro- 

 fessional entomologists ! Look at the beautiful (?) specimens offered 

 thereat Id. and 'h\. apiece, and then I think you will be convinced that 

 the original colour in most cases must have been tlie result of pigment 

 which has lost (probably by decomposition or decay, i.e. bleaching) its 

 power of absorption and now reflects (almost) white light, and cannot 

 have been produced l)y diffraction from surfaces which have remained 

 quite unchanged. The isolation of this pigment can only mean the 

 separation of the substance which possesses this selective power over 

 light, unchanged, and still in a condition to exercise this power, and 

 therefore remaining the same ''colour" that it was before isolation. 

 This obviously depends upon the action — chemical and mechanical — 

 of the solvent. The dye " magenta " is, in its dry state, a lovely 

 metallic green. Water does not change it, but alcohol dissolves it at once, 

 and we get the lovely dye ; crush the dry mass into a very fine powder, 

 and you will find that when you have done so you will have a pink, 

 almost magenta, dust. Many otlier substances present two colours accord- 

 ing to the way in which they are examined (dichroism) — c.(j. the platino- 

 cyanides. I have now before me a specimen of this compound of 

 yttrium. It presents the exact effect which I3r. Riding notices with 

 the scales of Thecla riihi, though much exaggerated. By reflected light 

 it is a brilliant green, but by transmitted light a good red. In both 

 instances there can be no doubt but that the red is the true colour. The 

 green is the effect of the dry pigment by way of reflection. I find it 

 hard to believe that the insects which we have dried as far as possible and 

 which have remained for years in our cabinets practically unchanged, owe 

 their colour to retaining water in the form of thin films, alternating 

 with chitin, kc, the removal of which could be the only (to my mind) 

 possible cause of such alteration of form as would pxit an end to the re- 

 fractive power of the scales. From thin films of chitin and air, the air 

 could not be removed; yet we know that were we to leave these 10 or 20 

 years' old insects exposed to light, they would very (piickly lose the colour 

 which absence of light has })reserved, and we always try and get them as 

 dry as possible before we put them away. Of course, I do not deny that 

 there are amongst the Lei)idoptera colours Avhich ai'e the result of 

 refraction, but it is my firm opinion that, as a rule, the colours are due 

 to '' pigment," which could be isolated if we could find a solvent which 

 would extract it without alteration. The (piestion of the minute 

 quantity is practically nothing. It would be interesting if some one 

 with leisure would write a short paper on the trade show-cases, 

 stating, if possible, how long these attractive specimens have been ex- 

 posed to light, and what has been the result on each species. I may 

 further add that simple cliitiu is clear, and transmits or reflects all 

 light like glass ; while miiiutely cellular chitin, being permeated by 

 air, &c., in a fine state of division, might appear to be what we call 

 white, just as snow, being in itself i)erfectly transparent, yet, being 

 permeated by air, appears white. Do these while scales show this 



