\ 



TRANSACTIONS OP THE SECTIONS. 



81 



and is read most conveniently througli this window, being illuminated by a beam 

 of light constantly directed upon it from a lamp hanging on end of the declination 

 axis, as will be afterwards explained. 



Between the fixed casting which forms the end of the telescope-tube and that 

 which revolves in it is another metallic circle cut into 3G0 teeth on edge, and 

 with 90 holes drilled accurately |^ on face ; into the teeth on edge is geared a screw 

 which is mounted on fixed casting, one revolution of which is of course equal 

 to an angular movement of 1°. 



In the other (outer) movable brass circle is mounted a steel pin working up 

 and down in a small cylinder ; this pin, being pressed down by a small spiral spring, 



Line of rays from 

 objective. 



Kg. 1. 



23 



I 



For position-circle. 



For bright field illumination 

 of micrometer. 



Portion of limb of transparent 

 Dosition-circle. 



position-circle. 



\v 



enters into one or other of the 90 holes in the intermediate circle and thus clamps 

 the whole eye end to the intermediate circle, in which condition a slow motion is 

 obtained by' the endless screw. When it is desired to move the eye cud through 

 a large angle, the rack-and-pinion tube is grasped by the band ; 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 movable circle, and allows it to be placed in any 



