one cm. square are superposed and cemented with an interposed air-film, and the piece is fixed against, say, the lower portion of the vertical slit. The liquid is, of course, continuous through the upper and uncovered portion of the slit; but, in the lower portion, the air-film separates the two semi-cylinders of liquid. Light passing through one of these semi-cylinders is thus incident on the second medium, the film of air, and when the angle of incidence is sufficiently large, the beam undergoes total reflection. In order that all the rays might undergo total reflection simultaneously, it is necessary that the incident beam should be parallel.
Adjustment for rendering the beam parallel.—The diagram represents a section of the liquid semi-cylinders: the one to the left we shall call P, that to the right Q.
Suppose O to be the principal focus of P; light diverging from a source O, would emerge into the air-film as a parallel beam, and would be focussed by Q at an equal distance, O′ to the right of the air-film. This will be all the more exact if a narrow diaphragm cut off all but the central rays. To render the beam incident on the air-film parallel, it is therefore only necessary to bring gradually the source of light near the cylinder, till an image is formed at an equal distance on the other side, the distances being measured from the axis of the cylinder. It will be observed that the semi-cylinder