

Matter moving through the Aither. ook 
had an arm for rotating and also a pointer passing over the 
scale referred to. 
In the adjustments 2 was moved until, when the trough 
was turned completely round, the ray as seen on a “white 
mark did not shift. Water which had been heated to drive 
out air and prevent minute bubbles forming in it and upon 
the mirrors and thus causing diffused light was then flowed 
into the trough until it covered the analysing and polarizing 
systems. This usually caused a shift of the rays, and 2 was 
again adjusted until the spot of light remained fixed when the 
trough was rotated. 8 was then adjusted until the return 
rays ~ passed through the analyser so as to give a uniform 
field of view when examined directly with the eye through 
a small circular aperture or by means of the telescope 14. 
The light after its passage through this 30 metres of water 
appeared of a beautiful light-green tint. With the mica 
sections removed the nicols were adjusted for extinction, 
which was fairly complete. ‘The sensitive strip 9 was then 
thrown in and rotated to extinction, and then turned through 
45° so as to bring its principal axes at 45° to the principal 
plane of the analyser. 10 was then placed in position and 
turned until the field on each side was of the same intensity 
as that of the sensitive strip. The eye thus saw the field of 
view illuminated uniformly with green light in the neigh- 
bourhood of this strip. The slightest trace of double refr action 
in the direction desired would at once make itself evident 
in the relative increase or diminution of the light from the 
strip. 
The conditions of maximum sensibility in photometric 
comparisons, namely a vanishing line and a uniform field, 
were thus attained. A small piece of glass compressed ver- 
tically to the slightest degree with the fingers placed after 
the polarizer 4 showed a sharp change of intensity at this 
bounding line. A match could be immediately obtained by 
rotating “the compensator 10. By noting the position of the 
pointer -for a match and then shifting the same until such a 
change could just be detected, a measure of the sensibility of 
the system could be obtained. This angle was found to be 0°-2 
under favourable conditions. At each observation the sensi- 
bility was determined. A match was obtained with, say, the 
trough in the meridian at noon, this was then turned through 
96° into the direction of the earth’s orbital motion. The 
position of the return image at the polarizer was noted, and 
if it had shifted in any way it was brought back by the 
adjustment of 2 into its initial position and then the field of 
view examined. In no case could a change be observed, i. ¢. 
