110 PROCEEDINGS OF THE ACADEMY OF [Feb., 



In studying the luster and iridescence shown by various natural 

 objects, it must first be considered to what causes they may be due, 

 which include: 



Simple reflection, as in the case of minute pyrite crystals sometimes 

 used in jewelry. 



Refraction and dispersion, generally accompanied by internal 

 reflection and sometimes by absorption, which account for the bril- 

 liancy and colors shown by dew drops, frost crystals and most cut gems. 



Scattering of light from microscopic particles, to which the blue 

 color of the sky is due. 



Polarization, which frequently accompanies the other phenomena, 

 but is rarely responsible for natural colors. If translucent tourmaline 

 enclosed in quartz or mica happens to be examined in the polarized 

 light from the sky at right angles to the direction of the sun's rays, 

 it may act as an analyzer, and if the film of the including mineral 

 between it and the source of light be of suitable thickness, show bright 

 colors. 



Diffraction, or the interference resulting from fine, uniformly spaced 

 lines or dots, which has been credited with being the cause of many 

 iridescent effects, with which it rarely has anything to do. The small 

 crustacean Sapphirina, which is said to sparkle like a gem when swim- 

 ming in the sun light, has a shell covered with fine markings similar 

 to those on Pleurosigma angulatum, which no doubt cause these 

 brilliant colored reflections by diffraction, and the chatoyance of star 

 sapphires and cat's-eyes may be due largely to diffraction resulting 

 from the symmetrically arranged inclusions. It must not be over- 

 looked, however, that to produce color effects by diffraction, the light 

 must come from but one direction, and the color will vary through the 

 entire spectrum with changes in the angle of incidence. 



There is one more cause to which these effects may be attributed, 

 and to which investigation will show that practically all iridescence 

 is due, and that is the interference produced by reflection from thin 

 films, which can be advantageously studied with the micro-spectro- 

 scope. Such interference colors generally show dark bands in the 

 spectrum, one in the lower order colors produced by thin films and 

 two or more as the films become thicker so that additional wave 

 lengths interfere. For comparison, records should be made of the 

 spectra of all the brighter colors, which can be done by observing them 

 in the "Newton's rings" produced between two surfaces of glass or 

 by blowing a bubble of melted glass until it bursts when the thin edges 

 will answer the same purpose. 



