
Lies 
a 
Matter moving through the Aither. 327 
number of years ago in experiments on the double refraction 
of light propagated at right angles to the lines of force in a 
magnetic field*. Each showed the black cross in certain 
azimuths between crossed nicols. In order to use this portion 
of the glass only, diaphragms 3 mms. to 4 mms. in aperture 
were placed at each of the ends of the cylinders along the 
optic axis of the system. One 2 linder was then inserted 
and adjusted so as to give the most satisfactory match ot the 
half-shade. The aperture was sufticient for obtaining a close 
setting, and the field could be made of nearly uniform inten- 
sity so that a vanishing-line was approximately realized. 
The slightest contact of the fingers, however, usually dis- 
turbed the match, in consequence of temperature changes. 
This match could be seen slowly to recover itself on removal 
of the fingers. Also the slightest flexure in the process of 
adjustment made itself at once manifest. This entire system 
was placed upon the large trough, and observations, with the 
polarizer at 45° to the vertical, made when the same was 
rotated. At first irregular changes in the match were 
observed, but these were found to be due to a shift of the eye 
and head of the observer. To eliminate this a seat was 
mounted so that the observer could move undisturbed with 
the trough, the head being steadied by means of a clamp. 
In this way no change in the field could be observed except 
in a very few instances. ae were attributed to an acci- 
dental shift of the eye or to slight temperature changes. 
Observations were made a noon and at 6 P.M. in the early 
part of December. Az this latter time occasional change 
could be detected similar to those which had been noted 
several times in the noon observations. The difficulty in 
maintaining the conditions for so high a sensibility as was 
attained, rendered the fictitious effects quite possible. When 
the greatest care was exercised no change could be detected. 
Under the most favourable conditions a rotation of the com- 
pensator of 0°2 approximately could be detected. This 
corresponds, as shown above, to a change in the relative 
retardation of 6° x 10-° 2 for the mean portion of the spec- 
trum, A=*000055 em. The total path in the glass was 
AA*5 ons: ., and hence the total number of waves is 
445% 1°77 
000055 — 
Thus the fraction of the total becomes 
6°5 x 10-°= (1°43 x 10°) =4°5 x 10-4, 
which is the greatest difference in velocity or in index 
* Phil, Mag. p. 342, Oct. 1897. 
= 143 x 10°. 
