F 
7 
————s eT. Se el! 

_ Matter moving through the Ather. B25 
increment in phase which represents the sensibility of the 
experiment in terms of the excess of the index above unity. 
This excess of index is } while the index is 4, hence our 
limit should be four times larger or 31 x 107. "The greatest 
change which oo be expected is the difference between 
unity and Waien where v is orbital velocity and V light- 
velocity or 

9 
= 
1 = 4(10-4? = 5 x 107%, 
or about 1600 times greater than the smallest effect which 
could be observed. 
The effect of a change in the frequency of the order ue 
on the index of the moving molecular vibrations relatively 
to the zther impulses in the direction of motion, is far too 
small to be observed. Thus the index of water for frequency 
31X10" is 1-334 and for 6°9x 10" is 1:341. This gives 
for a fractional increase in frequency of 4% a fractional in- 
crease in index of 0:007 x 4. Hence the fractional increase 
in index due to a change of frequency of order 0°5 x 10—* is 
O-55eh0=2 
4y 10" 
while the smallest observable change was 7°8 x 107). 
ee Oo xo 
Experiments with Glass. 
Two different arrangements were tried with glass. In the 
first a large slab of “optical”’ glass (crown) was cut length- 
wise and the edges of the two halves ground square. These 
were then cemented together side by side so as to give 
approximately square end-surfaces for grinding and polishing. 
These end-surfaces were “ built up” in the usual way so as 
to ensure a “flat” surface. These two prisms were then 
cemented end to end, giving a prism 42 ems. long with 
polished ends 10 cms. by 3 ‘8ems. These end-surfaces were 
silvered and a strip at the bottom and top of each removed. 
The system was then mounted on a support within the 
trough so that light from the polarizer could pass in and be 
reflected backwards and forwards until it passed out through 
the unsilvered space at the other end, where it was again 
reflected back into the prism by a concave mirror 3m. radius 
of curvature, approximately. After the same number of 
reflexions it passed out to one side of the entering ray and 
was received by the half-shade analysing system alre: ady 
