390 Prof. D. B. Brace on the 



about through 180°, the first passage would make the 

 emergent vibration along A/B/, since now the propagation 

 is against the drift. On reflexion the final vibration would 

 be along A/'B/'. Thus, on reversing the optical system, we 

 double the effect. Hence we are entitled to reflect the ray 

 backward and forward, compensation being attained when 

 the total distance in the liquid in one direction is equal to 

 that in the other. Such an arrangement contains at once the 

 possibilities of far greater sensibilities than any previous 

 attempts. 



The same mounting for the optical system was used as in 

 my former experiment " on the Double Refraction of Matter 

 moving through the iEther"*. The optical system, however, 

 was somewhat different. 



Instead of using plane-reflecting mirrors, as before, which 

 allowed the beam of light to spread too much, with the con- 

 siderable distances gone over, concave reflectors mounted 

 with adjusting screws were used instead ; their centres of 

 curvature being approximately at the axis of rotation of the 

 mount (fig. 3). The distance between the two sets of mirrors 

 at the end of the trough was 410 cm., and it required some 

 ■60 to 70 pounds of the oil to fill the trough. This arrange- 

 ment conserved the light without allowing it to spread out 

 continuously, which would have required a much larger 

 trough and consequent amount of liquid, This would have 

 produced greater distortion in the ray by its passage through 

 larger portions of the liquid. A brass trough whose section 

 was like the lower half of an octagonal cylinder was mounted 

 within the wooden trough, used in the experiment referred to. 

 This was just large enough to carry at its ends three mirrors 

 of two inches aperture arranged together with their centres 

 at the points of an equilateral triangle with its vertex down. 

 In order to avoid depolarization from reflexion and double 

 refraction, the polarizing and analysing systems were mounted 

 in the bottom of brass L tubes, which could be partly im- 

 mersed in the oil. The vertical axis of the first L was coin- 

 cident with the axis of rotation of the trough. A circular 

 beam of sunlight from a heliostat H, converging at the axis 

 of rotation E, was reflected downward by the prism R, 

 mounted on an universal carrier fixed to an arm I, coaxial 

 with the axis of rotation. The prism T reflected the ray 

 horizontally through the polarizing nicol P, thence through 

 the thin cover-glass C put into the liquid to the first concave 



* Phil Mag. April 1904, p. 317. 



