324 Prof. D. B. Brace on Double Refraction in 
From the above data we may calculate the least change in 
the index which could be observed if the water had become 
doubly refracting. Jf @ is the angle which the plane of 
polarization makes with one of the principal axes of the mica 
then the component vibrations or the principal axes of the 
resultant ellipse in the quarter-wave plate are in the ratio 
of tan@to1. For small angles then the ratio of the change 
Pe . 
of phase to the total or i 38 proportional to the angle @. 
Thus 1° rotation of the mica gives 
ge es 
Usage Mies oy 
but 16! of the quarter-wave plate was equivalent to 5° of the 
compensator, and as 0°-2 rotation of the latter could be 
detected, this reduces to 
16?) i gully Mie hie 9 
se ae a EOE 
approx. for green. The total path of the light in the water 
was 2856 cms. Taking its index as 1°33, the number of 
waves is 
= 0 Xho 
2800 X 1Ee on : 
Scan ae x ALO", 
As 6x 10-5 of a single wave could be detected, the fraction 
of the total would be 
i 
Oa 04 sx 5 LO = 7S 5e. LOG: 
This represents the greatest difference in velocity or in index 
between the two components which could exist referred to 
that of water * for green light, \=*00005 com. 
Mascart t has shown that in the case of water under com- 
pression the increment in the excess of the index above unity 
is nearly proportional to the increment of its density. If in 
the movement of matter through ether an increase in density 
in this direction took place, producing a change in’ the 
natural frequency of the molecular systems similar to that 
which occurs in glass, say, then, to determine how great it 
might be from these results, it is necessary to measure the 
* For carbon bisulphide Rayleigh obtained the corresponding limit 
of 4x10—" for yellow light. His retardation was calculated from Wer- 
theim’s results. This checks with the data obtained above as 200 gms. 
Ap 
12000 
gave 2°°5, hence 25 gms. would give 0°31 or a 0 instead of 
which he gives. 
+ Optique, t. iii. p. 613. 

