Sub-stage Condenser.
Polarising Prism, fixed in sub-stage below.
Fig. 184.—Diagrammatic arrangement of the Polarising Microscope.
In Sub-stage: P, polarising prism; C, sub-stage condenser on stage; M, mineral or crystal. On nose-piece: O1, objective, 4⁄10-inch; A, analysing prism.
In Draw-tube: O2, 2 or 3 inch Objective; H, Huyghenian eye-piece.
For the purpose, screw on a low-power objective to the end of the draw-tube ([Fig. 184]).[31] As the light requires to be passed through the crystals at a considerable angle, a wide-angled condenser should be employed, but it need not be achromatic. The objective most suitable is a 4⁄10-inch, of ·64 numerical aperture, but a ¼-inch of ·71 numerical aperture, or a 1⁄3-inch of ·65 numerical aperture, will answer the purpose equally well. As the whole of the back lens of the objective should be visible through the analysing Nicol prism, the back lens of the objective must not be too large; thus a ½-inch of ·65 numerical aperture will not be so effective. The analysing prism may be placed either where it is in the drawing, below the stage, or above the eye-piece. It works equally well above the objective, the position it ordinarily occupies in the microscope.
For the draw-tube a 2-inch objective and a B Huyghenian eye-piece answers very well. Before screwing the objective on to the end of the draw-tube centre the light in the usual manner, the Nicol’s being turned so as to give a light field, then screw the objective on to the end of the aperture, and put the crystal on the stage, rack down the body so that the objective on the nose-piece nearly touches the crystal, then focus with the draw-tube only. The sub-stage condenser should be racked up close to the underside of the crystal.
Opticians, however, have more recently furnished a special form of microscope (The Petrological Microscope, [Fig. 79], p. 112), for the use of those students who may desire to prosecute so fascinating a study, and determine the optic axial angles of crystals.