238 PROCEEDINGS OF THE AMERICAN ACADEMY 



Since science is so far advanced, that for every color produced by 

 interference of light the distance of the lamellns is known by calcula- 

 tion, it would be possible to give the exact figures of the distance for 

 every color observed on insects. The wings of some insects show 

 interference colors only for a certain time (Chrysopa, Agrion), as long 

 as the membranes of the wings are soft and not firmly glued together. 

 Later those wings become simply hyaline. On other insect wings 

 those colors remain during lifetime and persist even after death. 



Secondly, interference colors are produced by many very fine lines 

 or stria3 in very near juxtaposition. Such colors are easily observed 

 by looking in an oblique direction towards a glass micrometer, even 

 not a very finely divided one. Some forty years ago Mr. Barton, a 

 manufacturer in London, used to make iridescent buttons called iris 

 buttons. There were only thirty to forty impressed strite on a square 

 line. But each adjacent square line had the strice in another direc- 

 tion. The fine longitudinal and transversal lines of the Lepidopte- 

 rous scales seem to serve admirably well to produce the brilliant effect 

 of color-changing butterflies. But there must be something more 

 present, as most of the scales of Lepidoptera are provided with simi- 

 larly fine lines, and only comparatively few species change colors. I 

 remark purposely that the lines in the color-changing scales are not 

 in nearer juxtaposition. The explanation of the fact given a century 

 ago by Rosel,* stating that both sides of the lines (like small prisms) 

 were differently colored, was due to an optical illusion, explained by 

 the insufficient power of the non-achromatic microscopes at that time. 

 There may be a way of explaining this kind of iridescence not yet used 

 by naturalists; I mean calculation. The late Mr. Nobert, of Greifs- 

 wald, the unrivalled maker of the well-known test-plates, which con- 

 tain bands differing by the number of lines, had so far advanced that the 

 last band has lines with less than one 100,000th of an inch distance, 

 where the delicate lines of Diatoms are separated from each other by 

 one 50,000th of an inch. Some twenty years ago there did not exist 

 microscopes strong enough to see those lines, and it was doubted by 

 the French Academy if they were really present. Mr. Nobert, as 

 accomplished in mathematics as in mechanics, proved by calculation 

 based upon the interference colors produced by those lines, the space 

 between them. The result agreed perfectly with his previous asser- 



* A. J. Rosel, Monatliche Insecten Belustigung, 1755, vol. iii. p. 256, 

 pi. 44. 



