THE VARIATION OF ANGLES OBSERVED IN CRYSTALS. 
4G5 
and ensures that the trough shall l)e so placed as to have one of its faces 
joerpendicular to the collimator tube. 
The collimator being fixed, it may l)e desired to rotate either the crystal holder or 
the telescope while keeping the other parts fixed, or to rotate either of them t(\gether 
with the scale. For this purpose tlie usual clamping screws which bear upon the 
axle are dispensed with, and a circular metal disc, 15 centims. in diameter and 
1 millim. thick, is provided, whicli may Ije clamped to the goniometer staiid by one 
screw, to the telescope by a second screw, or to the crystal holder by a third screw. 
The disc is itself in rigid connection with the divided circle. Thus any two parts of 
the instrument may be coupled together, and it may be used eitlier as a goniometer 
or a spectrometer. The telescope is clamped to the stand by a fourth screw with 
lever head. 
Both telescope and collimator, as well as the eye-pieces, are mounted in collars 
provided with three screws, by which they may be fully adjusted. The telescope is 
also provided with a rack and pinion, so that it may, when used as a microscope, be 
moved towards or from the axis, and thus be focussed upon ciystals immersed in 
liquids of different refractive indices. For the measurement of crystals growing in 
their solutions the crystal is fixed in a small clip made of stout platinum wire or 
ribbon ; as the crystal grows the clip becomes enveloped by it and affords an 
absolutely rigid holder. 
In addition to the ordinary eye-pieces, it was found necessary at an early stage of 
the research to provide a sj^ecial eye-piece (fig. 2) for measuring the displacement of 
an image in any required direction within 
the field of view of the telescope. During 
the growth of a crystal the measurements 
can only be conveniently made in a single 
zone, and it is inadvisable to alter the 
adjustment of this zone during a series of 
observations. But, clearly, variations of 
angle may be due to the displacement of a 
face not only in the direction of a zone but 
out of it. 
If a face has been adjusted so that the 
image O (fig. 2) of the square collimator 
signal is on the intersection of the cross 
wires, and if during the gfowtli of the 
crystal the face alters its position slightly 
so that the image moves to P out of 
the horizontal zone CC, it will be 
sufiicient to measure the distance OP, and the angle which the line OP makes 
with the horizontal cross wire, without moving the crystal at all. For this 
YOL. CCII.-A, 3 O 
