EartJis Motion on the Rotation of a Refracted Ray. 595 



system is reversed in the direction o£ " drift," line 1' would 

 be the azimuth after the first plate and line 2' after returning 

 through the second plate. Thus, the total change in the 

 azimuth is equal to the angle between 2 and 2', and so on 

 for any number of pairs. It is evident, from the figure, 

 that, neglecting the effect of " drift," rays of all colours will 

 emerge from the system with their absolute directions of 

 vibration the same as when incident on the first plate. 

 Hence, if we start with our nicols crossed, the insertion of 

 these pairs of plates will not affect the final direction of 

 vibration of the rays, and we shall have perfect compensation 

 of the rotation, as well as of the rotary dispersion. Elliptic 

 polarization may of course occur to a greater or less extent, 

 due to imperfect annealing. 



The arrangement, as finally adopted to utilize these various 

 ideas, is shown in fig. 2. I is the radiant (a Nernst filament 



Fiff. 2. 



of 110 volts), N is a nicol for polarizing the light before 

 entering the half-shade system, in order to reduce the diffused 

 light arising from internal reflexion within the polarizer P 

 proper of the half- shade system PS, M and M' mirrors 

 silvered on the front surface, L a lens forming a conjugate 

 image of the radiant I on the analyser A, S the sensitive- 

 strip, 1, 2 and 1', 2' the two pairs of wedges, T a small 

 telescope of 2 or 3 diameters, and C the reading circle 

 carrying the analyser and provided with a micrometer-screw. 

 The sensitive-strip S, which I have already described *, 

 consisted of a plate of Iceland spar 0*15 mm. thick and so 

 mounted within a cell containing carbon disulphide that its 

 optic axis was perpendicular to the ray and covering one 

 half of the field of view. The micrometer tangent-screw was 

 provided with a drum so that it could be read directly. The 

 entire system was mounted on a wooden support, so that the 

 axes of the ray were in a horizontal plane, and this was 



* Phil. Mag. Jan. 1903, p. 161. 



2 S 2 



