287 
HerschelVs Telescope . 
The great mirror which, by proper methods, was 
brought to the lower part of the tube, is made of metal, 
49 1-2 inches in diameter ; but the concave part, or polish- 
ed surface, is only 48 inches in diameter. Its thickness 
is 3 1-2 inches ; and, when it came from the cast, its 
weight was 2118 pounds, of which a small quantity must 
have been lost in polishing. An iron ring, 49 1-2 inches 
in diameter, within 4 inches broad, and 1 1-8 inch thick, 
with three strong handles to it, goes round the mirror, 
and a flat cover of tin is made to correspond to this ring 
that the mirror may be preserved from damp ; and, by 
an easy contrivance, it is taken off and fixed on at pleasure., 
At the upper end, the tube is open, and directed to the 
part of the heavens intended for observation, to which the 
observer’s back is turned, and, he standing on the foot- 
board visible in the plate, looks down the tube, and per- 
ceives the object by rays reflected from the great mirror, 
through the eye-glass at the opening of the tube. Near 
the place of the eye-glass is the end of a tin pipe, into 
which a mouth-piece may be placed ; so that, during an 
observation, a person may direct his voice into this pipe, 
whilst his eye is at the glass. This pipe is 1 1-2 inch in 
diameter, runs down to the bottom of the tube, where it 
goes into a turning joint, thence into a drawing tube, and 
out of this into another turning joint, from which it pro- 
ceeds by a set of sliding tubes towards the front of the 
foundation timber. The use of this tube is to convev 
the voice of the observer to his assistants ; for at the last 
place it divides itself into two branches, one going into the 
observatory, the other into the workman’s room, ascend- 
ing in both places through the floor, and being terminated 
in the usual shape of speaking-trumpets. Though the 
voice passes in this manner through a tube with many in- 
flections, and not less than 115 feet, it requires very little 
exertion to be well understood. 
