
iim. 



Matter moving through the A&ther. 323 
the quarter-wave plate this match was destroyed, but by 
rotating the compensator this could again be obtained. In 
this way the retardation of the compensator could be at once 
determined in terms of that of the quarter-wave plate. Thus, 
arotation of 5° ofthe compensator corresponds to 16’ of that 
of the quarter-wave plate. It was found that the rotation of 
the compensator was proportional to that of the quarter-wave 
plate approximately for these small angles. 
A further comparison was made with the vertical crown- 
glass strip. This was 13 mms. wide and 2 mms. thick. The 
quarter-wave plate was removed and this strip inserted 
instead and a setting made with the compensator. On 
adding 200 gms. a match was obtained on rotating the com- 
pensator through 2°-5. From this can be calculated the 
relative retardation produced in glass per unit weight and 
unit width. Another comparison was made with white light 
from the acetylene flame direct by removing both strip 
and compensator and inserting the micrometer and _hori- 
zontal glass strip in addition to the vertical glass strip. When 
the clamp for producing flexure was screwed up a horizontal 
black band appeared between the two cross-wires. For one 
flexure, where the band was quite distinct, 500 gms. on the 
vertical glass strip gave a reading of 36 on the micrometer- 
screw and 200 gms. gave 14, thus showing the proportion- 
ality. A mov ement of the cross-wires, “iust sufficient to 
observe a shift, gave a reading of 12, which was the sensi- 
bility of the system for that flexure. On releasing the screw 
until the flexure was so far reduced that the band was barely 
visible,z;20G gms. gave a shift of 23 divisions and 100 gms. 
gave 11 divisions as near as could be observed, and this was 
the smallest wei ight which could be observed to produce any 
double refraction. A direct shift of the cross-wires gave 
13 divisions as the sensibility. Using direct white light and 
the sensitive strip and compensator 0°1 rotation of the 
latter could be detected, thus giving it a sensibility of 
zee 0-1=8 oms., or 12°5 times that of the band under 
similar conditions of light intensity and adjustment*. With 
greater intensity and more careful adjustment higher sensi- 
bility could be obtained by both methods. In fact, Rayleigh, 
using lime-light and a black band, has been able to detect a 
weight of 25 gms. on a vertical glass strip 15 mms. wide, or 
a sensibility over four times as great as that obtained above 
- with the acetylene flame and a black band. 
* A comparison with a Bravais sensitive-tint biplate gave 200 times 
the sensibility for the sensitive strip. 
