Lacroix 1 Axial Goniometer .— Winchell. 
81 
A 
c 
—? 
Fig. 2. 
Suppose that on trial with a thin section of muscovite it id- 
found that the distance 2sinE is divided by the revolution 
of the graduated rim B, fig. 1, into 1,430 parts. As these parts 
have no known value it is necessary to compare them with a 
standard. Such a standard is found in a mineral whose optic 
angle has been previously determined by other means. For 
instance, if the optic angle 2E in another specimen of musco¬ 
vite is known to be 71°, then E in the same is 35° 30'. 
Let the unknown distance between the loci of the optic axes 
in this muscovite be represented by 2d, and let the unknown 
ratio between its units and those of the number 1,430 be rep¬ 
resented b}^ M. Then 
On measuring 2d in this muscovite it may be found to be 
1,525. Hence 
log M dog 767- log 35° 30'; 
log M = 3.12085. 
Therefore, in the original specimen, 
log sinE — log 715 - 3.12085 1.73346. 
E = 32° 46* 31" 
2E = 65° 33' 2" 
The optic angle in muscovite may vary 30°. 
In makiiig such measurements it is necessarry to use always 
the same objective, and to employ the same constant M ascer¬ 
tained for the microscope in use, which may be kept in its- 
logarithmic form. 
As the apparent size of the interference figure depends on 
the position of the Bertrand lens, it is necessary also to have* 
