# Page:Collected Physical Papers.djvu/77

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COLLECTED PHYSICAL PAPERS

A beam of light is refracted from the given substance into air, and the angle of incidence gradually increased till total reflection just takes place. From the critical angle i thus determined, the index is found from the formula ${\displaystyle \mu ={\frac {1}{\sin i}}}$.

The necessary appliances for the determination are: (1) a hollow glass cylinder, which may be a beaker, for liquids (or a stoppered phial for volatile substances), or two semi-cylinders of the given solid; (2) a circular wooden table graduated into degrees and capable of rotation round a vertical axis. The table has at the centre a raised circular platform carrying an index. The platform can revolve round the common vertical axis independently of the table. When the platform is clamped down, the circle and the platform revolve together round a common axis.

The apparatus may be used for the following investigations:—

(1) Determination of the indices of solids and liquids.
(2) Variation of the index with the strength of different solutions.
(3) Variation of the index with temperature.
(4) Determination of the indices for the different rays, and of the dispersive power.

1. Determination of the index for liquids

A vertical, rectangular plate is suspended so as to divide the liquid in the glass cylinder into two halves. A narrow central and vertical slit is cut in the rectangular plate. Two thin microscope cover slips about