REFLECTING POWERS OF SELENIUM 
133 
backed with silver, is substituted for the crystal at X. The abso- 
lute reflecting power of glass backed with silver being easily found 
in physical tables, it becomes a simple matter to translate relative 
reflecting powers into absolute ones. On account of the loss in 
light resulting from dividing the beam, and using fairly high 
magnification it was not found possible to extend the results very 
far toward either the red or the violet end of the spectrum. In- 
dividual sets of observations were somewhat difficult to repeat 
with any great consistency, but by making many different set- 
tings, and using several different crystals it is felt that the mean 
results are correct to somewhere between 5 and 7 per cent. 
The extensive original data will not be presented here. The 
final mean values are shown in graphical form in the curves of 
figure 2. The curves are largely self-explanatory. The data of 
Foersterling and Freedericksz, 3 and of Pfund 4 are recorded for the 
R% 
30 
20 
70 
400 500 Afa) 600 700 
Figure 2. 
sake of comparison of my results with the results of other obser- 
vers on crystalline selenium in the form of cast, polished plates. 
The essential point to emphasize is that here we have double reflect- 
ing powers and that in previous work we have single reflecting 
powers. That former values are not exactly half way between 
these present double values is of no great moment. An exact mean 
would imply that the crystals were all lying on their sides, and hap- 
hazardly arranged. If any crystals were inclined to the reflecting 
surface of the plate, then the smaller reflecting power would pre- 
dominate. In fact recently Grippenberg 5 has concluded that in the 
process of crystallization of selenium plates, most of the crystals 
are formed “end on” to the plane surface. This agrees well with 
the results shown in figure 2, as previous values are distinctly below 
the average positions of my results. 
University or Iowa. 
o — o Pfund 
X X Foe rster/m Q-pre'ede/'/cksz 
+ 1 S/eq - B/ec tr/’c Vector || 
□ □ Sffq-B/eefe/c Vector X 
1 
Noziz:; 
j ~~ + — -] 
- + 
o~~ 
JHq. o~0“' 
— Or^H 
p. 6 < 
T * 
3 Foersterling and Freedericksz, Ann. der Phys., 43, p. 1227, 1914. 
4 Pfund, Phys. Zeit., 10, p. 340, 1909. 
5 Grippenberg, Phys. Zeit., 22, p. 281, 1921. 
