the Trigonometrical Operation. 153 
With regard to thefe eye-glaftes, it is only neceflary here to 
mention, that thofe of the leaft magnifying powers were found 
both in day and night obfervations to anfwer the beft. 
In the focus of the eve*glafs of the lower telefcope there 
are only two wires croffing each other in acute angles, which 
are vertical, inftead of being placed at right angles, horizon- 
tally and vertically, as was the ancient method. Since the 
lower telefcope never moves through more than a few degrees 
of a vertical arc, the wires require little or no adjuftment. 
Neverthelefs this was provided for, by allowing room for a 
fmall circular motion of the end-piece, which, when adjufted, 
is then fattened by its proper fcrews, and never afterwards 
needs any alteration. 
By referring to the middle part of Plate VI. two reprefen- 
tations of the eye end of the upper telefcope will be feen, with 
the eye-piece removed. Five wires are (hewn in this end, 
namely, two that interfed each other in acute angles, fimilarly 
to thofe in the lower telefcope; and three that lie horizon- 
tally or parallel to each other. Four of thefe, viz. the two 
that form the acute angles, and the two extreme horizontal 
wires, are fixed in the focus of the eye-glaftes to the farther fur- 
face of a thin brafs Aide, fuppofed to be feen through the out- 
ward brafs, and therefore (haded more dark than the reft. This 
Aide, as will be conceived, lies neareft the eye, and is move- 
able from right to left, and, vice verfa , horizontally, for the 
adjuftment of the line of collimation, by the infertion of a 
fmall mill-head key, on a fquare pin fitted to receive it, and 
fecured by a focket on the right hand fide. The fifth or mid- 
dlemoft horizontal wire is attached to the neareft fur face of a 
fteel Aide, that lies contiguouAy to, but beyond the former. 
It is made of one entire thick piece, like thofe of the micro- 
Vol. LXXX. X fcopes, 
