41i SUMSIARY OF CURRENT RESEARCHES RELATING TO 



waves of light are large in proportion to them, and so the liglit-wavcs 

 are reflected as if from a theoretically ihxt surface. But something more 

 than a polished glass is needed to reflect much light, for most of the 

 light passes through the glass ; something lum-transparent must bo 

 bc^hind the glass. In the common mirror it is a mercury amalgam; 

 in the butterfly's silvery scale it is a layer of cavities tilled with air. 

 This layer of cavities is not transiiarent for the same reason tliat 

 ground glass is not. If we ti'eat the scale with chloroform it has an 

 analogous effect to that of treating the back of a common mirror with 

 nitric acid, thus dissolving oft' the amalgam. In both cases a non- 

 transparent body is converted into a transparent one, and a mirror, 

 which, whatever be the materials of which it is made, if approximately 

 lierfect has a silvery appearance from the amount of reflected light, 

 is reduced to a slightly reflecting surface. But let the scale dry 

 again from its bath, as Fischer ajiparently did not do, and the mirror 

 will again appear. Both silvery and milk-white colorations are 

 therefore only optical cftects produced by reflected light. 



Still another kind of appearance is seen in the scales of Haplia 

 and of Entbnns. These scales are brilliantly coloured, yet their 

 colour is in the one case entirely lost, in the other case greatly 

 changed by wetting with almost any liquid, but when redried the 

 colours reappear with all their previous brilliancy. This coloration 

 also resists all forms of bleaching. It must therefore be produced by 

 some decomposition of light. Whatever acts upon the light must bo 

 within the scale, not uj^on the outside, for all those scales which 

 remain perfectly sealed, so that the liquid does not enter them, retain 

 their colour even when surrounded by liquid. This proves that the 

 colour is not due to external striation, where such exists. The finer 

 striation of the scales of Entimus is evidently internal, from its rela- 

 tions to the differently coloured internal cavities of the scales. Besides 

 this striation the interior of the scale is evidently filled with a pith- 

 like substance into which liquids enter with equal readiness in all 

 directions ; this pith-like portion apparently has some direct influence 

 upon the production of the coloration, for wherever it is injured or 

 has shrunk away from the basal end of a scale there is no longer 

 coloration in that place. Perhajis it is a necessary filling to cause 

 the strife to refract the light, the same as air-cavities arc necessary as 

 a backing to produce the silvery colour in the scales of Lepidoptera. 

 The strias themselves are very fine, but whether they are the causes 

 of colour is hard to determine without more accui-ate instruments of 

 measurement than the author had at his command. As near as ho 

 could determine they are 0*0008 to 0-0009 mm. apart. The wave- 

 length of a ray of light from Frauenhofer's A line of the spectrum 

 is, according to Willigen, 0"0007G092 ram., and the wave-length at 

 the H, line is, according to the same authority, 0* 00039713 mm. ; the 

 difibrence being 0' 00036379 mm., or the difterence of wave-length 

 between violet and red light. To determine the 2)lace in the spectrum 

 to which the stria) of these scales correspond would require, of course, 

 much finer measurements. 



The author adopts Ilagen's division of colours into " o])tical " and 



