the trouble entailed in following the deflected image in the prism-method is avoided. As the cylinder revolves round, total reflection will take place four times, twice
Fig. 14. The Spectrometer circle. There is a sliding tube with a slit at one end. The distance of the slit from the centre of the cylinder is adjusted till a sharp image is formed at an equal distance on the screen to the right. A vertical plate with a slit and air-film is suspended dividing the cylinder in two halves.
from the air-film surface of each semi-cylinder. The source of light and a screen being now fixed at the extremities of a diameter passing through the centre of the circle, the distance of the slit is adjusted for rendering the beam incident on the air-film parallel, in the manner already described. When the angle of incidence reaches the critical value, the image on the screen is suddenly extinguished. In the centre of the screen there is a circular aperture, behind which the eye itself may be placed to receive the light directly. When white light is used, as in the determination of the indices for the different rays, the image is cast on the slit of a small direct vision spectroscope, and the spectrum observed.
A portion of light may be allowed to pass through the upper part of the slit, where the liquid is continuous, and the corresponding portion of the image will never
