Measurement of the Angle of a Prism

(a) Keeping the prism fixed. - Adjust the prism so that an image of the slit can be seen distinctly by reflexion from each of two of its faces, and its edge is parallel to the axis of rotation of the telescope.

Adjust the telescope so that the image of the slit reflected from one face coincides with the vertical cross-wire, and read the verniers. Move the telescope until the same coincidence is observed for the image reflected from the second face, and read again.

The difference of the two readings is twice the angle required, provided the incident light is parallel.

(b) Keefing the telescope fixed. - Move the prism until the image of the slit reflected from one face coincides with the vertical cross-wire, and read the verniers for the prism.

Turn the prism until the same coincidence is observed for the other face, and read again.

Then the defect of the difference of the two readings from 180° is the angle required.

Verify by repeating the measurements.

Experiments.

(1) Verify the law of reflexion.

(2) Measure by methods (a) and (b) the angle of the given prism.

Enter results thus:

(1) Displacement of telescope: 5°42'       24°0'15"
    Displacement of the prism: 2°51        12°0'0"
(2) Angle of prism
By method (a): 60°7'30" 60°7'50" mean 60°7'40"
By method (b): 60°8'15" 60°7'45" mean 60°8'00"