MANIPULATION OF THE TELESCOPIC ALIDADE 101 
The cross-hair ring, or reticle, is placed so as to be at a prin- 
cipal focus of the object-glass as well as at a focus of the eyepiece. 
Its position is indicated by four screws or capstans on the outside 
•of the telescope tube. The cross-hairs are spider webs, or very 
fine platinum wires, almost invisible to the unaided eye. They 
are generally four in number and are fastened immovably with 
shellac to the brass ring before it is inserted in the telescope. 
Two of the hairs cross the center of the ring at right angles to 
each other; their purpose primarily is to define the line of sight. 
The other two are parallel to one of these and spaced equidis- 
tant on either side of it; they are the stadia hairs and are used 
primarily for measuring distances. In the Gale design of ali- 
dade, none of these hairs are adjustable with respect to each 
other, but the whole reticle may be moved by means of the four 
screws which hold it in place. 
It is very important for purposes of adjustment to understand how 
the capstan screws control the movement of the cross-hair ring. . . 
. . The holes in the telescope through which the screws pass are not 
threaded; on the contrary, they are a little larger than the screws, so 
that when the latter are loose the whole ring may be turned slightly 
by moving the four capstan heads simultaneously around the outside 
of the telescope until one cross-hair is vertical and the other hori- 
zontal. When the capstan screws are tight, each screw presses a 
curved washer (shown in the photograph) against the outside surface 
of the telescope. When one screw is loosened and the opposite screw 
tightened, the whole ring Is drawn toward the tightened screw (since 
the holes in the shell of the telescope are smooth) until the loose screw 
and its washer are brought into contact again with the outside of the 
telescope. Notice that before tightening one screw the opposite screw 
should be loosened, otherwise the ring cannot move and the screw- 
thread may be stripped. By loosening the lower screw and tightening 
the upper screw, the whole ring may be drawn upward, or by reversing 
the process it may be drawn downward. Likewise by working the side 
screws in a similar manner, it may be drawn to one side or the other. 
All this may be done without turning the ring, i.e., one hair may be 
kept vertical, the other horizontal.® 
® J. C. Tracy, loc. cit., p. 550. 
