g 86 Dr. Herschel's Description of a 
At the opening of the telescope, near the place of the eye- 
glass, is the end of a tin pipe, into which at the time of obser- 
vation a mouth-piece may be put, which can be adapted, by 
drawing out, or turning sideways, so as conveniently to come 
to the mouth of the observer, while his eye is at the glass. 
This pipe is \\ inch in diameter, and runs down to the bottom 
of the telescope, to which it is held by proper hooks, that go 
into the tube, and are screwed fast at the inside. When it is 
arrived as near to the axle AB, fig. 34. as convenient, it goes 
into a turning joint ; thence into a drawing tube, and out of 
this into another turning joint ; from which it proceeds by a 
set of sliding tubes towards the front of the foundation timber. 
The mechanism of the first turning joint and short sliding 
tube, as well as the next turning joint, is executed in brass, as 
represented in fig. 39. The tube a is the continuation of the 
pipe which comes down from the observer ; at b and c it is 
turned about in an angle, but the part b and c consists of a 
double brass tube, one of which may be turned within the 
other, b de, is an arm which has two pivots, one at b, the 
other at c ; the part d is put through, and pinned to a fasten- 
ing at the tube, where it is also permitted to turn about if re- 
quired. When the telescope is lifted up, the pipe a b c turns 
upon the pivot b, and within the pipe bee; which also turns 
upon the pivot c ; so that a be may come at rectangles to 
b c e, when the telescope is turned up to the zenith. At the 
same time c e sliding in fg, will be drawn out, since be is not 
in the axis of the vertical motion, which lies in A B, fig. 34. 
but turns in a small arch about it. The point c will not only 
be drawn back, but will also be lifted up, and therefore a se- 
cond turning joint, bg , becomes necessary, which is of the 
