320 Prof. D. B. Brace on Double Refraction in 
The total-reflecting prism 2 was carried by a universal 
mounting passing through a rod forming the prolongation of 
the axis about which the system rotated. By properly 
shifting 1 and 2 the ray 2-3 could be brought exactly in the 
axis of rotation, so that when the trough was rotated the 
return ray at 9 remained at a definite point in the field of 
view. 3 and 4 were then shifted until the ray passed 
through them symmetrically. Any change in the direction 
of the incident ray at 1 would of course cause a shift, but by 
properly regulating the heliostat this could be avoided. 
However, with such a long optical lever slight irregularities 
might occur after a rotation, but these were always com- 
pensated for before observing the field of view by adjusting 2 
until the beam of light occupied the exact position it did 
previous to rotation. 
The polarizing nicol was either one with ends normal to. 
the ray, or, if of the ordinary type, mounted in a cell with 
thin cover-glass ends so as not to affect the ray when the 
system was under water. The analysing nicol was a Glan- 
Thompson of 15 mms. aperture. The analysing and polar- 
izing systems together with the prisms and lens were 
mounted within tubes to prevent access of the water and 
upon a common cross-piece fastened to the trough. By ad- 
justing 8 the cone of rays could be sent into the analyser 
symmetrically so as to fill completely the field of view. The 
principal planes of the nicols were crossed and at 45° to the 
vertical plane. A metal diaphragm was placed lengthwise 
between the entering and the emerging rays and between 
the mirror 5 and the polarizing system so as to prevent 
scattered light reaching the analyser. 
The following delicate method, a detailed description of 
which I give elsewhere*, was used for observing the slightest 
trace of double refraction. 9 was a thin strip of mica which 
I will designate as the “sensitive strip”’ of order ;d9 or 
0°0012 mm. thick, cemented with Canada balsam between 
two thin cover-glasses without double refraction, the latter 
being cemented to a brass ring carried by an arm extending 
from a collar slipping over the brass containing-tube of the 
nicol. This collar carried an arm with the scale divided into 
some 60 divisions representing half degrees. 10 was a 
similar thin section of mica of order 7s approx., cemented 
similarly and covering nearly the entire aperture of the 
nicol 11. This system, which I will designate as the ‘* com- 
pensator,’’ was mounted on a collar slipping over the nicol 
between the collar and the strip of the first system. This 
* Phys. Rey. Feb. 1904. 
