900 
MR. A. E. TUTTON ON AN INSTRUMENT FOR GRINDING SECTION- 
are concluded the table light is switched permanently on for the operation of 
grinding. 
Adjustment of the Instrument. 
It will have been observed that every part of the instrument is provided with its 
own means of adjustment, in order to be capable of accurately performing the duties 
relegated to it. 
The adjustment of the telescope exactly perpendicular to the vertical axis of 
rotation is performed in the usual manner, after clearly focussing a distant object, by 
so manipulating the two adjusting screws arranged in the vertical diameter of the 
collar that the images of ihe cross-wires reflected in succession from the two surfaces 
of a small mirror silvered on both sides, and carried upon the ciystal holder instead 
of a crystal, or from twn brilliant parallel faces of an opaque crystal, can be made to 
coincide with the wires themselves as seen through the telescope. The telescope 
then recjuires to be further adjusted for azimuth ; that is, its axis must be directed 
riorht at the axis of rotation, so as to intersect the latter. In order to effect this the 
crystal holder is replaced by the short pointed rod seen to the extreme left of the 
base-board in fig. 1. This is centred by use of the centering movements, so that 
when observed through the telescope, arranged as a microscope by addition of the 
movable lens, the point appears stationary upon rotating the axis. The lateral 
adjusting screw of the telescope collar is then so manipulated, if alteration is 
necessary, that the point occupies the centre of the field. 
The adjustment of the collimator is then readily effected by manipulating the screw 
of its collar so that the image of the slit, illuminated by the goniometer lamp, seen 
directly by arranging telescope and collimator in the same straight line, is clearly 
focussed, perfectly upright, and is bisected at its narrowest central point by the 
horizontal cross-wire. 
The adjustment of the grinding surface parallel to the plane of the axes of the 
telescope and collimator, and therefore perpendicular to the vertical axis of rotation, 
is achieved m the following manner. It is first ascertained that, for all positions of 
the collimator and telescope along the circular guiding bed, their plane remains 
perpendicular to the axis of rotation which carries the crystal. The telescope is then 
fixed at the extreme right of its guiding bed, almost touching the pillar, and the 
collimator brought to the nearest end of its guiding arc, the angle between the two 
optical tubes being thus about 120°. A glass cube or prism, of about 1 inch side, and 
of which two faces are ground quite plane, and are accurately inclined at 90°, is next 
required. By goniometrically testing a few glass models of cubes or prisms, or a 
number of rectangular reflecting prisms, one can usually be found which exhibits two 
faces inclined at 90° to within a very few minutes. A small cubical glass ink-well 
was found to possess two faces inclined at 89° 58', and answers the purpose admir¬ 
ably. If available, a large natural crystal which exhibits two such faces free from 
