Page:BraceRefraction1904.djvu/11

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^[1]. 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 cylinder was then inserted and adjusted so as to give the most satisfactory match of the half-shade. The aperture was sufficient for obtaining a close setting, and the field could be made of nearly uniform intensity so that a vanishing-line was approximately realized. The slightest contact of the fingers, however, usually disturbed 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. These were attributed to an accidental shift of the eye or to slight temperature changes. Observations were made at noon and at 6 P. M. in the early part of December. At 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 compensator of 0°.2 approximately could be detected. This corresponds, as shown above, to a change in the relative retardation of $6.5\times 10^{-5}\lambda$ for the mean portion of the spectrum, λ=.000055 cm. The total path in the glass was 44.5 cms., and hence the total number of waves is

${\frac {44.5\times 1.77}{.000055}}=1.43\times 10^{6}$.

Thus the fraction of the total becomes

$6.5\times 10^{-5}\div (1.43\times 10^{6})=4.5\times 10^{-11}$,

which is the greatest difference in velocity or in index

1. Phil. Mag. p. 342, Oct. 1897.