112 PROCEEDINGS OF THE ACADEMY OF [Feb., 



at about wave length .588 micron. The position and width of this 

 band correspond to that in the spectrum of blue of the third order, 

 and accordingly may be accepted as indicating a film about .58 

 micron in thickness, the film being assumed to be of air or gas enclosed 

 within the chitine of the scale, and not consisting of chitine itself, in 

 which case it would be over a third thinner. 



The reason for this assumption lies rather in analogy with the results 

 obtained from study of the scales of beetles than in the behavior of 

 the butterfly scales themselves. It is well known that if a portion of 

 a diamond beetle is mounted in balsam, the iridescent reflections from 

 the scales are more brilliant than when examined dry, but if the scales 

 be scraped from such a beetle and then mounted in balsam, most of 

 them will become perfectly colorless, w T hile here and there may be one 

 still retaining its iridescence. A further examination will show that 

 such colored scales are perfect, while the others have all been more or 

 less broken in removing them, indicating that the iridescent film occu- 

 pied an internal interval, which was penetrated by the balsam in all 

 broken scales, but remained unimpaired as long as no such penetration 

 could be effected. 



If the iridescent feathers of birds be next examined, the resemblance 

 of their brilliant metallic reflections to those of butterfly scales would 

 seem to justify the expectation that this is due to a similar cause, but 

 this predication will be contradicted by an examination of their 

 spectra. For instance, the spectrum from a ruby-tinted, iridescent 

 humming-bird feather shows no dark bands, but on the contrary a 

 single bright band, including part of the red and orange, and reference 

 to the record of spectra previously made proves that there can be no 

 color resulting from interference caused by a single film that could 

 produce such a spectrum. We can, however, readily understand the 

 probable cause of this color by first considering to what the colors 

 in precious opal are due, a subject that has been elucidated by high 

 authorities. 



In selecting opals for this investigation, those presenting patches of 

 uniform color are to be preferred, and the best will be found among the 

 so-called black opals which have recently become popular as gems. 

 I have examined a large number of these, both in polished specimens 

 and microscopic sections, as a result of which it is evident that they 

 consist of precious opal, which by some convulsion of nature has been 

 shattered to fragments, which were subsequently re-cemented by a 

 further deposition of opal of a gray or black color, which serves to 

 render the reflections more brilliant by cutting out all extraneous 



