=64. Kinds of Polariscopes.=—The simplest form of a polarizing apparatus consists of two nicol prisms, one of which, _viz._, the analyzer, is capable of rotation about its long axis. The prolongation of this axis is continuous with that of the other prism, _viz._, the polarizer. The two prisms are sufficiently removed from each other to allow of the interposition of the polarizing body whose rotatory power is to be measured. For purposes of description three kinds of polarimeters may be mentioned. 1. _Instruments in which the deviation of the plane of polarization is measured by turning the analyzer about its axis._ Instruments of this kind conform to the simple type first mentioned, and are _coeteris paribus_ the best. The Laurent, Wild, Landolt-Lippich, etc., belong to this class. 2. _Instruments in which both nicols are fixed and the direction of the plane of polarized light corrected by the interposition of a wedge of a solid polarizing body._ Belonging to this class are the apparatus of Soleil, Duboscq, Scheibler, and the compensating apparatus of Schmidt and Haensch. 3. _Apparatus in which the analyzer is set at a constant angle with the polarizer, and the compensation secured by varying the length or concentration of the interposed polarizing liquid._ The apparatus of Trannin belongs to this class. =65. Appearance of Field of Vision.=—Polarimeters are also classified in respect of the appearance of the field of vision. 1. _Tint Instruments._—The field of vision in these instruments in every position of the nicols, except that on which the plane of vibration of the polarized light is coincident with the three principal sections, is composed of two semi-disks of different colors. 2. _Shadow Instruments._—The field of vision in this class of polarimeters in all except neutral positions, is composed of two semi-disks, one dark and one yellow. As the neutral position is approximated the two disks gradually assume a light yellow color, and when neutrality is reached they appear to be equally colored. The Laurent, Schmidt and Haensch shadow and Landolt-Lippich instruments, are of this class. 3. _Striated Instruments._—In this class the field of vision is striated. The lines may be tinted as in Wild’s polaristrobometer or black, as in the Duboscq and Trannin instruments. The neutral position is indicated either by the disappearance of the striae (Wild) or by the phenomenon of their becoming continuous. (Duboscq, Trannin.) =66. Character of Light Used.=—Polariscopes may be further divided into two classes, based on the kind of light employed. 1. _Instruments which Use Ordinary White Light._—(Oil lamp, etc.) Scheibler, Schmidt and Haensch. 2. _Instruments Employing Monochromatic Light._—(Sodium flame, etc.) Laurent, Landolt-Lippich, etc.