ted on a suitable vertical stand. Anarm moving round this circle carries a telescope DE whichpoints to the centre of the circle. The position of this armcan be read by a vernier attached to it. The weight of thetelescope is balanced by a counterpoise hung on the other endof the arm and shewn at F.… … Fig. 119. behind by a light and the rays diverging from the slit fall onthe lens G. Since the slit is at the principal focus of the lens 170 LIGHT. [CH. VIII the rays emerging from the lens are parallel. These parallelrays fall on the prism K, placed at the centre of the circle. Tofind the angle of…
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