THE ROYAL ARTILLERY INSTITUTION. 
199 
The angle-finder may be considered to have two sets of parts essen¬ 
tially different, namely, those on which the accuracy of the angles depend, 
and those which are merely used for convenience. 
The first comprise only the object glasses and the wires. The second 
include the eye-pieces, the case to protect the short telescope, the tangent 
screw, &c. 
Many people attribute equal importance to all portions of the instru¬ 
ments, and if the limb fits loosely, or if the exterior screws shake, think 
that the angles may be affected. This is erroneous; every single screw 
that can be seen from the outside of the instrument could be removed 
without the accuracy being at all impaired. 
Wire Piece of the Large Telescope. 
This is composed of a stout cylinder, a small moveable plate, and 
platinum, or gold wire. 
The screws fastening the wires are keepered, that is, smaller screws 
are driven through them. 
The wire plate is fastened to the wire cylinder by two keepered screws. 
There are two additional screws which appear also to fasten the wire 
plate, but these are only partly used for this object, as they have play, 
their real purpose is to aid in the collimation. 
There are two side screws extending from the wire piece to the outside 
of the instrument, these are only used for focussing. 
The wires of the main telescopes have never yet been broken or in¬ 
jured. 
Should this happen, if an instrument maker performs the repair, he 
should remove the object-glass and eye-piece, remove the screws fasten¬ 
ing the wire piece, and force the wire piece out by ramming a stick in at 
the object-glass side, he will put on new wires, and if these are found not 
to be in collimation, he must collimate that is, bring the wires exactly 
in the axis of the telescope, by shifting, tentatively, the collimating plate. 
If the wires are injured on service , 
Proceed to remove the wire-piece as above. 
If a bit of a surveyor, let the officer put on the cobweb of a field-spider 
instead of wires ; if he does not understand this operation, let him twist 
unspun silk, a very thin fibre of tow, or any other thin fibre, round the 
screws, securing the ends with sealing wax. He should not attempt to 
regulate the collimation, but refasten the wire-piece; the observations 
(after testing) will be perfectly accurate, although the instruments will 
not be as convenient to use with unskilled men. 
Object Glass. 
The object-glass, that is the large glass of the main telescope, has not, 
to my knowledge, become shaky in any instrument during the last two 
years. 
A shake will be detected by pressing the fore-finger lightly on the 
glass and endeavouring to move it. The amount of inaccuracy which 
a shake is liable to produce can be detected by taking observations when, 
the object glass is pushed into two extreme positions. 
