568 
Proceedings of the Royal Society 
prisms, I turn the system of flint-glass prisms until the spectrum 
appears to he broken off at a definite Fraunhofer line, measure 
the angle z, and thus obtain — 
Sint,-—', 6 = V-i, x = /usin0. 
Of course one can use flint-glass prisms other than right-angled ones, 
and, in fact, for measures of quartz, I have had flint-glass prisms 
made by Steinheil in Munich, in which i was only a few degrees. 
In a plate of quartz cut perpendicularly to the optic axis, one can 
easily determine in this way the refractive indices of the ordinary 
and extraordinary ray by interposing in front of the eye a Nichol’s 
prism in the proper position. In fresh quartz plates I obtained 
almost exactly the same values as Rudberg : — 
Rudberg. 
B 
C 
D 
E 
F 
G 
1-54090 
1-54181 
1-54418 
1-54711 
1-54965 
1-55425 
1-54990 
1-55085 
1-55328 
1-55631 
1-55894 
1-56365 
Quincke ^ 
zur Axe. 
1-54108 
1-54207 
1-54412 
1-54710 
1-54966 
1-55365 
1-54987 
1-55065 
1-55338 
1-55622 
1-55892 
1-57166 
Quincke Quartz _L zur Axe 
c 
1-54022 
1-54092 
1-54318 
1-54575 
1-54845 
1-55246 
1-54880 
1-54955 
1-55245 
1-55533 
1-55801 
1-56163 
Quincke Quartz _L zur Axe 
;> 
1-53958 
1-54087 
1-54335 
1-54649 
1-54868 
1-55243 
1-54789 
1-54933 
1-55199 
1-55508 
1-55758 
1-56192 
The differences of the individual measurements in different speci- 
mens I attribute to difference in the properties of the specimens of 
quartz themselves. 
In old quartz surfaces which have already been about twenty 
years in my possession, and in others which I have found in the 
