Fig. 29 illustrates in section the construction of a dichroscope. The instrument consists essentially of a rhomb of Iceland-spar, _S_, of such a length as to give two contiguous images (Fig. 30) of a square hole, _H_, in one end of the tube containing it. In some instruments the terminal faces of the rhomb are ground at right angles to its length, but usually, as in that depicted, prisms of glass, _G_, are cemented on to the two ends. A cap _C_, with a slightly larger hole, which is circular in shape, fits on the end of the tube, and can be moved up and down it and revolved round it, as desired. The stone, _R_, to be tested may be directly attached to it by means of some kind of wax or cement in such a way that light which has traversed it passes into the window, _H_, of the instrument; the cap at the same time permits of the rotation of the stone about the axis of the main tube of the instrument. The dichroscope shown in the figure has a still more convenient arrangement: it is provided with an additional attachment, _A_, by means of which the stone can be turned about an axis at right angles to the length of the tube, and thus examined in different directions. At the other end of the main tube is placed a lens, _L_, of low power for viewing the twin images: the short tube containing it can be pushed in and out for focusing purposes. Many makers now place the rhomb close to the lens, _L_, and thereby require a much smaller piece of spar; material suitable for optical purposes is fast growing scarce.