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.
67. Interchangeable Instruments.—Some of the instruments in common use are arranged to be used either with ordinary lamp or gas light, or with a monochromatic flame. Laurent’s polarimeter is one of this kind. The compensating instruments also may have the field of vision arranged for tints or shadows. Theoretically the best instrument would be one in which the light is purely monochromatic, the field of vision a shadow, and the compensation secured by the rotation of the second nicol.
The accuracy of an instrument depends, however, on the skill and care with which it is constructed and used. With quartz wedges properly ground and mounted, and with ordinary white light, polariscopes may be obtained which give readings as accurate as can be desired.