=71. Construction of Laurent’s Apparatus.=—The shadow polariscope invented by Laurent is constructed as follows: The polarizer is a special nicol which is not fixed in its position, but is so arranged as to be turned through a small arc about its axis. By this device, the quantity of light passing through it can be regulated, and the apparatus is thus useful with colored solutions which are not easily cleared by any of the common bleaching agents. The greater the quantity of light admitted, however, the less delicate is the reading of the shadow produced. The plane of polarized light emergent from this prism, falls on a disk of glass half covered by a thin lamina of quartz which thus divides the field of vision into halves. It is this semi-disk of quartz which is the distinguishing feature of the apparatus.[40] The polarized light thus passes without hindrance the half field of vision which is covered by the glass only, but can not pass the quartz plate unless its axis is set in a certain way. The field of vision may be thus half dark, or both halves may be equally illuminated or equally dark according to the position of the nicol analyzer which is freely movable about its axis and carries a vernier and reading glass over a graduated circle. The field of vision in the laurent may have any of the following forms.[41] Let the polarizer be first so adjusted that the plane of polarization of the transmitted pencil of light is parallel to the axis of the plate lying in the direction A B. The two halves of the field of vision will then appear equally illuminated in every position of the analyzer. But if the polarizing nicol be inclined to AB at an angle a, the plane of polarization of the rays passing through the quartz plate will undergo deviation through an equal angle in the opposite direction. [Illustration: FIGURE 35. FIELD OF VISION OF A LAURENT POLARISCOPE.]