89G 
MR. A. E. TUTTON ON AN INSTRUMENT FOR GRINDING SECTION- 
parallel to one of the tangent screws, and the other, of more complicated structure, 
is employed for the purpose of grinding a second surface parallel to one already 
ground. These will be described at a later stage. 
The telescope and collimator are carried upon rigid supports which slide upon a 
circular guiding bed, whose centre of curvature lies on the vertical axis of the 
instrument. The use of three columns instead of four for the purpose of carrying 
the circle and suspended axis is of advantage, inasmuch as it enables nearly 120° of 
guiding bed for the optical tubes to be employed, and permits of arranging them in 
the same straight line so as to directly*view’ the slit whenever desired. The sliding- 
bases of the supports for the optical tubes are maintained firmly pressed against the 
circular steel guides by means of slightly curved springs placed between the edge of 
the latter on the inner side and the rabbet of each sliding base. Both telescope and 
collimator are capable of being adjusted to the same horizontal plane, perpendicular 
to the axis of rotation of the instrument, towards which also the optical tubes can be 
precisely directed. For this purpose the main outer tube of each carries a collar, 
wdiich is screwed to the stouter collar of the support by means of two adjusting 
screw^s arranged near the extremities of the vertical diameter; these enable the 
altitude to be slightly varied ; a third adjusting screw on one side at 90° from the 
others enables adjustment for azimuth to be effected. Both telescope and collimator 
are capable of sliding in the outer tube, so as to be approached nearer to or receded 
from tlie crystal; they may be fixed in any desired position by means of split rings 
tightened round the outer tube of each, wdiich is also split for a short distance, by 
means of a tightening screw. The telescope is capable of accurate adjustment for 
parallel rays, the eyejDiece being carried in an inner draw^-tube ; the cross wires are 
placed in a short tube forming a continuation of the latter, to wdnch it is attached by 
means of a fine screws thread, which permits of the necessary focussing of the cross 
wdres. By means of a split-ring collar furnished wdth clamping screw’ and carrying a 
small projecting wmdge wdnch fits into a corresponding notch in the objective tube, the 
eyepiece may be fixed, after adjustment, so that the two clearly defined cross wires 
are respectively vertical and horizontal and a distant object is clearly focussed. In 
front of the objective a movable lens is carried, capable of rotation upon a pivot 
fixed to the objective frame, of such focus that ^vdlen rotated into position the 
telescope is converted into a low^-pow^er microscope, the focus of wdiich is occupied by 
the crystal, which is consequently well defined in the centre of the field. The 
collimator carries at the end of an inner draw-tube a slit of the form devised by 
Websky, produced by employing portions of two circular metal discs as jaws instead 
of straight edges. This form of slit combines the advantages of a narrow’ central 
portion, wdiich can be adjusted to a cross wire with the greatest accuracy, w’ith 
wider ends wdiich pass more iglit. The inner tube which carries the slit is provided, 
like the eyepiece tube, with a split-ring collar carrying a wmdge wdiich fits into a 
notch in the wdder tube carrying the collinifiting lens, so that the slit may be fixed 
