354 
MINUTES OF PEOCEEDINGS OF 
a large angle, the rack and pinion tube is grasped by the hand, and in doing so 
the hand almost necessarily grasps also a small steel trigger which lifts the steel 
pin out of the hole, frees the moveable circle, and allows it to be placed in any 
angular position. When the desired position is approximated, and the trigger 
relieved, the pin drops into the nearest hole, and the endless screw is then used for 
final setting. 
“ The diagram will, I think, explain the various matters of illumination. 
“From a lamp hanging upon the end of the declination axis is sent a beam of 
slightly divergent light through this axis, which is hollow; this slightly divergent 
beam is utilised for six different purposes, three portions of it being reflected out in 
different directions to illuminate portions of the declination circle, of which one is 
for a long reader for setting from eye-end, and the other two for micrometer 
microscopes subdividing the 10' division of circle into single 1" arc. 
“ None of these are shown in diagram, but the other three purposes for which 
the light is utilised—viz., for position circle, bright field illumination, and dark 
field illumination of micrometer—are shown. 
“ The position circle illumination is very simple. (See Fig. 1). A single reflector, 
R , attached to the inside of the tube, directs a constant beam of light on the back 
of the glass circle at P. 
“ The bright field illumination is effected by a very small central reflector, R' t 
which sends the light directly into the field of the micrometer. 
“ This method is, I believe, now generally considered to give the best results, 
and has, as far as I am aware, but one disadvantage—viz., that the arm which 
supports the small mirror produces a little diffraction, and consequently deterioration 
of definition. 
Fig. 1. Fig. 2« 
