PLATES AND PRISMS OP CRYSTALS OF ARTIFICIAL PREPARATIONS. 907 
any desired inclination to the plane of the other instead of being fixed at right angles, 
enables this to be carried out. It is only necessary to set the two motions parallel to the 
two faces of a prism of the same form, one on either side of the symmetry plane, and 
to rotate the segments by means of the tangent screws for the calculated number of 
degrees. The calculation is a very simple one, the total amount of desired rotation 
in the symmetry plane (the extinction angle) and the inclination of the two circular 
motions to that plane being known. In order to set the motions parallel to the two 
prism faces, it is sufficient to set one parallel in the usual manner to the lower 
tangent screw, then starting with the motions parallel, the indicator at zero, to 
rotate this lower motion about the upper for the number of degrees (read upon the 
small horizontal graduated circle which registers the rotation) corresponding to the 
known angle of the prism. In order to be able to carry out this adjustment easily, it 
is advisable to take somewhat more than the usual care to cement the crystal upon 
the holder so that the prism zone is as nearly as possible in the approximately correct 
position parallel to the axis of the holder, so that very little preliminary adjustment 
is necessary before rotating the segments for their calculated arcs. 
The same adjustment may be attained even more easily by employing this alter¬ 
native pair of circular motions in another manner. One of the prism faces is set 
parallel to the lower tangent screw, and the lower segment then rotated about the 
upper one, by means of the horizontal circle, for the number of degrees corresponding 
to the angle between the prism face in question and the symmetry plane, so that the 
plane of the upper circular motion will be parallel to the symmetry plane. The 
approximate preliminary adjustment of the prism zone parallel to the axis of rotation 
of the instrument is then rendered perfect by a few successive approximations with 
the two motions thus inclined. The axis of optical elasticity perpendicular to which 
a section is to be prepared may then at once be brought vertical with respect to the 
grinding plane by rotation of the upper segment for the number of degrees corres¬ 
ponding to the determined extinction upon the symmetry plane, that is, corresponding 
to the known deviation of the median line to be adjusted from the vertical axis of the 
crystal. 
The above four cases illustrate the possibilities of usefulness of the instrument, 
but it will rarely hapj^en that the more diflScult cases will have to be resorted to. 
Crystals will usually be found which exhibit primary faces which will enable the 
desired plane to be immediately set parallel to the grinding disc without any pre¬ 
liminary calculation. Even if such faces are only developed to the extent of a mere 
line, that is quite sufficient, for usually a reflection of the Websky slit will be afforded 
of sufficient brightness to enable the adjustment to be effected. The case of triclinic 
crystals is, of course, more difficult, and no general statement of their mode of treat¬ 
ment can be given ; the plan of operations must be thought out for each crystal. 
With the information afforded by stauroscopical and convergent light observations 
through the various pairs of faces, an approximation to the positions of the axes of 
5 z 2 
