Verification of the Law of Reflexion

This requires the table on which the prism is fixed to be capable of motion round the same axis as the telescope, and to have a vernier attached.

Adjust the apparatus so that the reflected image of the slit coincides with the cross-wire, and read the position of the telescope and prism. The slit should be made as narrow as possible.

If the instrument has two verniers for the telescope opposite to each other, read both and take the mean of the readings. Errors of centering are thus eliminated.

Move the prism to another position, adjust the telescope as before, and take readings of the position of the prism and telescope. Subtract these results from the former respectively. It will be found that the angle moved through by the telescope is always twice that moved through by the prism.