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I'srciiE. 



IJiily — Aus:"st iSS_^ 



claitstraria, which appendages owe 

 their silvery whiteness to air within 

 them. Again he mentions hairs whicii 

 contain air on spiders of tlie genera 

 Epcira and Thcridiitm. Then fur- 

 ther, when considering the insects. Lev- 

 dig writes that it is not difficult to see 

 that the silvery under surface of Hy- 

 dronu'tra pallidum is due to the pore- 

 canals being tilled with air. He goes 

 on to say " In a similar way the wings 

 of Notonecta glatica seem to enclose 

 air, and I suspect also that the white 

 color of the hairy powder of many 

 apJiidae and coccidac is brought about 

 by like causes." Further on he writes, 

 '■If one regards the color of scales it 

 can inhere as diffuse material in the 

 substance of the scale itself, or it ap- 

 pears under the form of molecular 

 pigment, whicli is deposited in the 

 cavities of the scales, or finally the 

 cavities are filled with air which gives a 

 snow-white appearance to the scale." 

 Again Leydig writes that when Fischer 

 says, in speaking of ''granulation-scales" 

 (i. e., such scales as those of Hop/ia 

 trifasciata)^ "that the 'upper or gran- 

 idatioii layer' dissolved visibly in water, 

 but quickly in alcohol or ether, and then 

 only the 'striate basal layer' remained, 

 the words show that he has certainlv 

 seen but incorrectly explained that 

 change which the scale undergoes upon 

 the loss of air, in so far as he assumed 

 a 'granulation laver' which dissolves in 

 water !" 



Leydig accounted for silvery glisten- 

 ing scales and surfaces, and for milk- 

 white coloration among insects, but he 



fails to account for the difierence between 

 these two kinds of coloration. The 

 white scales of Picris rapac and the 

 silvery scales on the under side of the 

 posterior wings of Argytniis. idalia 

 both contain no appreciable coloring 

 matter, and both contain air; both, too. 

 are simply milk-white by transmitted 

 light. The difference is that there must 

 be in tlie silvery scales a polished sur- 

 face towards the observer. Ground 

 glass does not appear silvery, but what 

 is the surface of the sinoothest polished 

 plate of glass but finely ground glass .^ 

 Ground glass differs from polished 

 glass only in degree : in ground glass 

 the scratches are so coarse and so 

 abundant as to turn most of the 

 light-waves into the glass again, where 

 thev are lost. In polished glass the 

 scratches are still present, but have 

 become so small that even the waves of 

 light are large in proportion to them, 

 and so the light-waves reflect as if from 

 a theoretically flat surface. But some- 

 thing more than a polished glass is 

 needed to reflect much light, for most 

 of the light passes through the glass : 

 something non-transparent must lie 

 behind the glass. In the common 

 mirror it is a mercury amalgam : in the 

 butterfly's silvery scale it is a la\er of 

 cavities filled \vith air. This layer of 

 cavities is not transparent for the same 

 reason that ground glass is not. If wo 

 treat the scale with chloroform it has 

 an analogous efiect to that of treating 

 the back of a conunon mirror with 

 nitric acid, thus dissolving oil' the 

 amalgam. In both cases a non-trans- 



