P acts as a collimating lens, and Q as the object-glass of an observing telescope. It will also be seen that the value of the index may now be obtained with fair accuracy from the distance OO′ (which is equal to four times the focal distance of the cylinder) and the radius of the cylinder.
As the lens is cylindrical, the source of light may be a line instead of a point. A narrow slit may be illuminated by sodium light for monochromatic measurements; or the slit may be illuminated by sun-light. It is sometimes more convenient to use the incandescent filament of a glow lamp as the source of light.
Method of experiment.—The cylinder containing the liquid with the interposed air-film is placed on the platform, the axis of the cylinder passing through the axis of the graduated circle. If the cylinder is kept fixed, and the direction of incident light changed, it will be observed that there are four positions, A, B, C, D, for total reflection, the angles AOB or DOC being each equal to twice the critical angle.
The source of light may be kept fixed, and the angle of incidence varied by rotating the central platform on which the cylinder with the interposed film is placed, till total reflection just takes place. The image produced remains fixed, till at the critical angle it suddenly disappears from the screen; this is a great advantage, as
