1911.] NATURAL SCIENCES OF PHILADELPHIA. Ill 



If the natural iridescent surfaces shown by many minerals be now 

 examined, similar interference bands will be noted in the spectra, 

 assuring us of the cause of iridescence and permitting the thickness 

 of the film to be calculated if the refractive index of the mineral be 

 known. This is shown particularly well on the iridescent surfaces 

 which sometimes appear on dendritic inclusions of magnetite in mica 

 from Delaware Co. Penna. One such surface of uniform pink color 

 gave a spectrum with two dark bands, centering at about wave lengths 

 .545 and .457 micron, corresponding to a red of the fifth order, and 

 indicating a thickness, if we assume the film to be a hydrate of the 

 composition of goethite, of about .53 micron or .000021 inch. In a 

 similar manner the colors reflected by scales of hematite, to which the 

 chatoyance of varieties of feldspar known as "sun stone" is due, can 

 be proved to result from the thinness of the films. The oligoclas 

 from Tvedestrand, Norway, frequently contains films of sufficient 

 size and uniformity of thickness to produce sharp interference bands. 

 Some hypersthene as well as other minerals showing aventurine 

 reflections contain similar inclusions, but not always of the same 

 mineral. The common aventurine or gold stone used in jewelry is an 

 artificial glass which owes its brilliancy to reflections from enclosed 

 crystals of metallic copper. 



There is no class of objects furnishing better examples of brilliant 

 iridescence than the scales of butterflies, and this is generally attributed 

 to the fine lines and markings with which they are covered, which cannot 

 be the case, as such markings could only cause color effects by diffrac- 

 tion, and the limitations to this cause have already been mentioned. 

 It is true that diffraction effects may be obtained from these scales 

 when held in a certain position with respect to the light, particularly 

 when they have been mounted on glass for examination by transmitted 

 light, but the colors thus produced which undergo changes according 

 to the angle of the illumination are not the characteristic colors of the 

 scales as seen in position on the insect, which are equally well shown 

 in diffused light. Furthermore, it will be found that in many butter- 

 flies the scales showing metallic colored reflections are not the ordinary 

 lined scales, but are apparently specialized for their purpose, showing 

 no fine markings, but merely rather coarse longitudinal corrugations. 

 When examined under the micro-spectroscope, most butterfly scales 

 are not sufficiently uniform in color to give pronounced interference 

 bands, but a darkening of the spectrum in some one position may 

 usually be noted, and in the particularly brilliant blue spots on the 

 wings of Papilio parts, a distinct black band is shown, having its center 



