A REDETERMINATION OF THE PRINCIPAL REFLECT- 
ING POWERS OF ISOLATED SELENIUM 
CRYSTALS 
L P. SIEG 
Some time ago, following the publication of a paper by one 
of my former students, Mr. C. H. Skinner, 1 I attempted to check 
his results by direct experiment. The paper referred to dealt 
with the optical constants of isolated selenium crystals. Of these 
constants, the reflecting power is the only one that oilers itself 
to an easy experimental test. It must be said that the results 
obtained by Skinner were obtained by indirect means, and since 
the equations used in reducing his results to optical constants 
were somewhat complicated, and rested on foundations none too 
sure, it was important for our own satisfaction at least, to make 
a direct test of their accuracy. The results of my experiments 2 
confirmed me in my belief that there were two principal reflecting 
powers to be expected; that with the incident light plane polar- 
ized with the electric vector parallel to the crystal axis, and that 
with the vector perpendicular to the axis. At the time of publi- 
cation, however, it was felt that the results should be looked on 
as merely preliminary in nature on account of the difficulty I 
had in setting up a satisfactory spectrophotometer for the small re- 
flecting surfaces available. 
Last fall I finally succeeded in devising a form of apparatus 
which proved to be highly satisfactory for the work in hand. 
With this apparatus I made a redetermination of the two prin- 
cipal reflecting powers, confirming in a general way my previous 
results. While the present results are not entirely free from 
error, they are so much more consistent than the former, that 
I feel considerable confidence in them. The same method of 
procedure is now being followed in the study of tellurium crys- 
tals. 
The arrangement of the apparatus is shown in elevation in 
figure 1. Light of the desired wave length from the slit of the 
monochromatic illuminator S is made parallel by lens L x . A 
portion of this beam is intercepted by mirror M x , and then made 
l Skinner, Phys. Rev., 9, p. 148, 1917. 
