In all cases there are two positions, differing by 180°, which give a minimum of light, and the two positions intermediate between these give a maximum of light. The extent of the changes thus observed is a measure of the completeness of the polarisation of light.
The two prisms mounted as shown in Figs. 179 and 179a constitute the apparatus adapted to the microscope. The polariser slips into place below the stage, and the analyser, with the prism fixed in a tube, is screwed in above the objective.
The definition is considered by some experimenters as somewhat better if the analyser be used above the eye-piece, and is certainly more easily rotated.
Fig. 180.—Prism mounted as an Eye-piece.
Method of employing the Polarising Prism ([Fig. 179]).—After having adapted it to slide into a groove on the under-surface of the stage, where it is secured and kept in place by the small milled-head screw, the other prism [Fig. 179]a) is screwed on above the object-glass, and thus passes directly into the body of the microscope. The light from the mirror having been reflected through them the axes of the two prisms must be made to coincide; this is done by regulating the milled-head screw until, by revolving the polarising prism, the field of view is entirely darkened twice during its revolution. If very minute salts or crystals are submitted for examination then it will be found preferable to place the analyser above the eye-piece, as in [Fig. 180]. Thus the polariscope is seen to consist of two parts; one for polarising, the other for analysing or testing the light. There is no essential difference between the two parts, except what convenience or economy may lead us to adopt; and either part, therefore, may be used as polariser or analyser; but whichever is used as the polariser, the other becomes the analyser.
Fig. 181.—More Modern Polariser and Analyser.
Opticians have their own methods of adapting the polariser and analyser to their several microscopes. Watson’s special form of apparatus is represented in [Fig. 181], the polariser being adapted to the sub-stage, and the analyser to screw into the objective.
Tourmaline.—A semi-transparent mineral, of a neutral or bluish tint, called tourmaline, when cut into thin slices (about 1⁄20-inch thick) with their faces parallel to their axes exhibit the same phenomena as the Nicol prism. The only objection to which is that the transmitted polarised beam is more or less coloured. The tourmaline to be preferred stops the most light when its axis is at right-angles to that of the polariser, and yet admits the most when in the same plane. Make choice of a tourmaline as perfect as possible; size is of less importance when intended for use with the microscope.