Page:Collected Physical Papers.djvu/60

40 in the following manner. Having adjusted the semi-cylinder for a given angle of incidence, the receiver was moved round till it responded to the refracted ray. Readings were taken first by placing the receiver at an angle less than the true reading and gradually increasing the angle till there was a response. The receiver was then placed at a greater angle, and the angle gradually reduced till the receiver again responded. In this way a series of readings for a particular angle of incidence was obtained. These readings were found fairly concordant, the maximum variation from the mean being not so great as 1°. One set of readings being taken on one half of the spectrometer circle; the cylinder was rotated in the opposite direction, and readings taken on the other side.