merwin: chromatic reflection in covellite 341 



PHYSICS.- — Covellite: A singular case of chromatic reflection. H. 

 E. Merwin, Geophysical Laboratory. 



To describe and to account for some striking color changes in 

 covellite when it is immersed in colorless inert liquids of different 

 refractions is the object of this study. 



Covellite, crystalline cupric sulphide, in finest powder is very 

 dark blue; brilliant crystal surfaces or polished plates, whether ob- 

 liquely or vertically illuminated, are lighter, and vary noticeably 

 in color with differences in crystallographic section. In obliquely 

 incident daylight a plate, parallel to the cleavage, immersed in 

 alcohol (refractive index, n = 1.36) appears brilliant purple, 1=* 

 in benzene {n = 1.50) it appears reddish purple, ^^ and in methy- 

 lene iodide (n = 1.74) it appears red.i° A plate perpendicular 

 to the cleavage changes only to purple in methylene iodide. 



Natural crystals have not yielded flakes thin enough to transmit 

 sufficient light for optical study but some synthetic crystals 

 prepared in this laboratory by E. G. Zies and by Eugene W. 

 Posnjak were found suitable for microscopic study. These 

 crystals are hexagonal scales or piles of scales reaching 0.5 mm. in 

 diameter, and 0.002 mm. in maximal thickness. The thicker 

 plates transmit scarcely perceptible amounts of light even when 

 illuminated by direct sunlight, the thinner ones are yellowish 

 green- to dark olive. By both a spectrometer eye-piece and a 

 prism monochromatic illuminator the yellowish green color was 

 found to be caused by less transmission of the red and violet than 

 of the middle of the spectrum. The fall in relative transmission 

 from green to violet — at least as far as 410/x/x — is not apparently 

 very great; it is greater in the red from about 630 to TOO/x/x mm. 

 The transmission is shown in a general way in figure 1 . 



Plates probably not exceeding 0.0005 mm. in thickness served 



1 The colors were compared with Ridgeway's standards. Although the stand- 

 ards are less brilliant approximate matches were made, (a) Between violet- 

 purple and true purple, (b) Near rhodamine purple, (c) Between spectrum 

 red and rose red. 



* E. Weinschenk prepared similar plates and noted the red color reflected 

 after mounting in Canada balsam. Z. Kryst., 17: 497. 1890. 



