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having an angle of 60°. I again found for one of the indices the same value, 1.04; and for the other, 1.15 instead of 1.19.

In order to control the results obtained by the prisms, I determined the indices by producing, by means of an aluminium lens, images of the lamp filament, and measuring their distances from the lens. The lens, which is plano-convex, has a radius of curvature of 6.63 cms., and an aperture of 6.8 cms. The slit of the wet screen is widened so as to form a circular opening 6 cms. in diameter; the lens is placed at a known distance ($p$ cm.) from the incandescent filament, and by means of the phosphorescent sulphide, the position of the conjugate images of the filament is determined. The following table gives the values of the indices found, both with the prism and the lens:—

Prisms. Lens. 27° 15′ 60° $p=40$ $p=30$ $p=22$ 1.85 {{{1}}}„ ... ... 1.86 1.91 1.91 1.68 {{{1}}}„ ... ... 1.67 1.66 1.67 1.48 {{{1}}}„ ... ... 1.50 1.44 1.48 1.40 {{{1}}}„ ... ... 1.42 1.42 1.43 1.36 {{{1}}}„ ... ... 1.36 1.36 1.37 1.29 {{{1}}}„ ... ... 1.36 1.31 1.37„ 1.19 1.15 ... ... 1.20 1.31„ 1.37„ 1.04 1.04 ... ... 1.20„ 1.31„ 1.37„