57 
§ 38 .] MICROSCOPIC INVESTIGATION OF CRYSTALS. 
stage, the circle being graduated in degrees, and polarising and analysing 
Nicol prisms ; the analyser being the upper nicol, for chemical work it 
is generally agreed that the upper nicol is best placed above the eyepiece, 
which also must have a graduated arc or circle. The eyepiece must be 
provided with cross hairs, at right angles. It is important, although not 
absolutely essential, that the fine adjustment be provided with a milled 
graduated head and mechanism estimating in parts of a millimetre the 
distance between the objective and the slide, so as to obtain the refractive 
index according to the method of de Chaulnes (see p. 58). The very 
highest class of instruments provide micrometer screws reading to 
a thousandth of a millimetre. An Abbe condenser which can readily 
be adjusted at different heights, and can easily be swung out of position, 
completes the essentials. 
The use that such a microscope can be put to is :— 
1. The measurement of angles. 
2. Ascertaining the refractive index or indices. 
3. Viewing interference figures. 
4. Ascertaining the “ extinction angle.” 
5. Observing the presence or absence of pleochroism. 
1. Angles. —The angles of any thin, well-formed crystal plate can 
be measured, by first adjusting the crystal in the exact centre of the 
field, and then adjusting one of the cross hairs by rotation of the eyepiece 
exactly parallel, and reading the graduated circle attached to the 
stage ; the latter is now rotated so that the other bounding edge is 
parallel with the same cross hair, the position of the graduated circle is 
noted, and the difference between the two angles is the angle sought. 
Or a goniometer eyepiece with fixed stage may be employed on the 
same principles. 
2. Ascertaining the refractive index. 
(a) Immersion method.—Crystals and transparent bodies viewed by 
the aid of the microscope are usually bounded by dark lines or bands, 
or are surrounded by a more or less coloured halo ; the width of these 
contours depends mainly upon the difference of refractive index and 
dispersion of the transparent solid and liquid. “ Contour bands appear 
when the refractive index is either greater or less than that'of the liquid 
in which the solid is immersed. As the index of refraction of the solid 
approaches closer and closer to that of the liquid, the dark bands de¬ 
crease in prominence, and finally vanish when both object and liquid 
have the same refractive index.” 1 
Theoretically, a colourless solid in a colourless liquid, dispersion 
and refraction equal, becomes invisible. These conditions are not met 
with in practice ; the object may become very faint in outline but, owing 
1 Elementary Chemical Microscopy, by E. M. Chamot. New York, 1916. 
