180 IOWA ACADEMY OF SCIENCE 



With this view in mind, some two years ago, the writer set 

 one of his students, Mr. C. H. Skinner, at work to determine 

 the optical constants by a katoptric method of an isolated cry- 

 stal of selenium. Although selenium is not regarded as a 

 metal, it has, nevertheless, optically speaking, many qualities 

 of a metal. The crystals are optically very dense and offer 

 brilliant reflecting surfaces. Skinner succeeded in proving 

 definitely that there were distinct differences in the optical 

 constants, depending upon whether the long axis of the hex- 

 agonal crystal was vertical or horizontal. This work is not 

 yet published in full, but an abstract has recently appeared 1 . 

 Some years ago Drude 2 and Midler 3 worked on isolated crystals 

 of antimony sulphide, and proved that from one cleavage plane, 

 (0 1 0) two distinct sets of optical constants were obtained. 

 Other work has been done, notably by G. Horn 4 , on the absorp- 

 tion of certain crystals, but the writer is not aware that any 

 work on elemental metallic crystals with the exception of a 

 determination of the absorption of bismuth and antimony by 

 the latter author, has been done. The reason for this is not 

 far to seek, for usually the crystals of metals are microscopic in 

 size. The difficulty in working with such small surfaces by 

 the ordinary methods led the writer to suggest this task of 

 obtaining a special method to another of his students, L. D. 

 Weld. The latter has succeeded admirably in this problem, 

 and a preliminary report of the work is presented to the Acad- 

 emy of Science at this meeting. 



Aside from the importance of crystals in yielding us 

 the true constants of various substances, there is good 

 reason for believing that the crystalline surface should be bet- 

 ter adapted for the purpose than any artificially formed pol- 

 ished surface. If this is correct, then even the cubic crystals 

 should be re-examined by this method. The one reliable trait 

 of all crystals seems to be their maintaining of constant angles, 

 and even if there seem to be striations on some of the surfaces, 

 it may be that these irregularities would not seriously affect 

 the optical constants. Then too where it is possible to obtain 

 a fresh surface by cleavage, one should expect the most relia- 



1 C. H. Skinner, Phys. Rev. 7, 1916, 285. 



2 P. Drude, Ann d. Phys., 34, 1888, 489. 



3 E C. Miiller, N. Yahrb, f. Miner. Beil. Bd., 17. 1903, 187. 



4 G. Horn, N. Yahrb. f. Miner. Beil. Bd., 12, 1899, 269. 



