12 CRYSTALLOGRAPHY. 
The wheel, 2, is connected with an axis which passes through the 
main axis (which is hollow for the purpose), and moves merely the 
parts to which the crystal is attached, in order to assist in its adjust- 
ment. The contrivances for the adjustment of the crystal are at a, D, 
c, d,k. The screws, ¢, d, are for the adjustment of the crystal, and the 
slides, a, 0, serve to centre it. 
To use the instrument, it may be put on a stand or small table, with 
its base accurately horizontal, and the table placed in front of a win- 
dow, six to twelve feet off, with the plane of its circle at right angles 
to the window. A dark line must then be drawn below the window, 
near the floor, parallel to the bars of the window, and about as far 
from the eye as from the window-bar. 
The crystal is attached to the movable plate i by means of wax, and 
so arranged that the edge of intersection of the two planes forming the 
required angle, shall be in a line with the axis of the instrument. 
This is done by varying its situation on the plate, or by means of the 
adjacent screws and slides. 
When apparently adjusted, the eye must be brought close to the 
crystal, nearly in contact with it, and on looking into a face, part of 
the window will be seen reflected, one bar of which must be selected 
for the trial. If the crystal is’ correctly adjusted, the selected bar 
will appear horizontal, and on turning the wheel 7, till this bar, as 
reflected, is observed to approach the dark line below seen in a direct 
view, it will be found to be parallel to this dark line, and ultimately to 
coincide with it. The eye for both observations should be held in 
precisely the same position. If there is not a perfect coincidence, the 
adjustment must be altered until this coincidence is obtained. Con- 
tinue then the revolution of the wheel n, till the same bar is seen by 
reflection in the next face, and if here there is also a coincidence of 
the reflected bar with the dark line seen direct, the adjustment is com- 
plete ; if not, alterations must be made, and the first face again tried. 
In an instrument like the one figured, the circle is usually graduated 
to twenty or thirty minutes, and, by means of the vernier, minutes and 
half minutes are measured. After adjustment, 180° on the are must 
be brought opposite 0, on the vernier, v. The coincidence of the Lar 
and dark line is then to be obtained, by turning the wheel 7. When 
obtained, the wheel 7 should be turned until the same coincidence is 
observed, by means of the next face of the crystal. If a line on the 
graduated circle now correspords with 0 on the vernier, the angle is 
immediately determined by the number of degrees opposite this line. 
If no line corresponds with 0, we must observe wtrich line on the 
vernier coincides with one on the circle. If it is the 18th on the 
vernier, and the line on the circle next below 0 on the vernier marks 
125°, the required angle is | 25° 18’; if this latter line marks 120° 2¢ , 
the required angle is 125° oo. 
In the better instruments other improved methods of arrangeme: t 
are employed ; and in the best, often called Mitscherlich’s goniometer, 
because first devised by him, there are two telescopes, one for passing 
a ray of light upon the adjusted crystal, having crussed hair lines in its 
focus, and the other for viewing it, also with a hair cross. With such 
an arrangement, the window-bar and dark line are unnecessary, the 
hair crosses serving to fix the position of the crystal, and the telescope 
that of the eye. If the crystal is perfect in its planes, and the adjust- 
