322 Prof. D. B. Brace on Double Refraction in 
there was still a match indicating no double refraction. 
Various positions were taken in and at right angles to the 
meridian with the same result. Hence, we may conclude 
that to this order of sensibility there is no double refraction 
in the water due to its motion through the ether. These 
observations were taken during the latter part of July 1903. 
It is evident that a rotation of the plane of polarization due 
to the earth’s field of force would not affect this match, as 
both portions of the field would vary in intensity by the same 
amount. To make sure of this the trough was rotated 
through 180° into the meridian so as to reverse the direction ; 
but no effect could be observed. It is evident that since the 
rotation due to a magnetic field is always in a definite direc- 
tion and independent of the direction of the ray, such a 
rotation of the plane of polarization would be reversed with 
respect to an observer moving with the trough. Hence 
this could not mask any effect due to double refraction. 
A. second check was made with a cell of turpentine 
1-6 mm. thick, whose ends were made with thin cover-glasses 
without double refraction, which would give a rotation of 
about 0°°5 , while if we take 0°°015 as Verdet’s constant for 
water and 0:2 as the earth’s field and a length of 30 m. we 
find about 0°15 for the rotation. On inserting this cell after 
the polarizer, no effect could be detected. 
In order to determine the relative retardation which cor- 
responds to a given rotation of the compensator, the polarizing 
and analysing systems were dismounted and placed on a 
support with their optic axes in line. The system was illu- 
minated by an acetylene flame, the light from which passed 
through green glass or celluloid of about the same tint as 
that obtained after passage through the water. The sensitive 
strip, compensator, a quarter-wave plate mounted on a ver- 
tical circle, and a vertical strip of glass capable of carrying 
a weight, and, in addition, a micrometer-screw carrying two 
horizontal cross-wires in front of a horizontal strip of glass 
held within a clamp so as to produce a flexure, were arranged 
to be placed in the path of the light. The order of the mica 
quarter-wave plate was found to be approximately 4 for 
green light, A=0:00005 em., by comparison in the usual way 
with a quartz or selenite wedge. With the nicols crossed 
and the plane of polarization at 45° to the vertical, the circle 
carrying the quarter-wave plate was adjusted until the light 
was extinguished and the mean of its positions for a number 
of settings noted. The sensitive strip was then thrown in 
with its axes at 45° to the plane of polarization, and after 
that the compensator which was set foramatch. By rotating 
