Figure 36. Laurent Polariscope.
11. Reading glass N, carried on the same radius as the mirror and used to magnify and read the scale:
12. Device F, to regulate the zero of the instrument:
13. Tube H, carrying a nicol analyzer and ocular O for defining the field of vision: This tube is rotated by the radial arm G, carrying the mirror and reading glass.
72 Manipulation.—The lamp having been adjusted, the instrument, in a dark room, is so directed that the most luminous spot of the flame is in the line of vision. An observation tube filled with water is placed in the trough and the zero of the vernier is placed accurately on the zero of the scale. The even tint of the field of vision is then secured by adjusting the apparatus by the device number 12.
73 The Soleil-Ventzke Polariscope.—A form of polariscope giving a colored field of vision was in use in this country almost exclusively until within ten years, and is still largely employed. There are many forms of tint instruments, but the one almost exclusively used here is that mentioned. A full description of their construction and manipulation is given by Tucker.[43] By the introduction of a third rotating nicol in front of the lens next to the lamp, the sensitive tint at which the reading is made can be kept at a maximum delicacy. These instruments are capable of rendering very reliable service, especially in the hands of those who have a delicate perception of color. They are inferior, however, to the shadow instruments in delicacy, and are more trying to the eye of the observer. The shadow instruments therefore, especially those making use of an ordinary kerosene lamp, have practically driven the tint polariscopes out of use.
The general arrangement of a tint instrument as modified by Scheibler is shown in [Fig. 37].
Figure 37. Tint Polariscope.