Forty-feet Reflecting Telescope. 383 
inclosed at the front and sides, a bar which shuts up the back 
after the observer is in his place, secures the whole in such a 
manner as to render it perfectly safe and convenient. 
There are two strong iron quadrants with teeth, at the sides 
of the seat, in which run two pinions fixed upon a bar, with a 
ratchet and handle at the end of it. By turning that handle, 
the seat is easily brought to an horizontal position, before the 
observer enters it ; or restored to it, when any considerable 
alteration in the altitude of the telescope renders a change 
necessary. 
The focus of the object mirror, by its proper adjustment, is 
brought down to about 4 inches from the lower side of the 
mouth of the tube, and comes forward into the air. By this 
arrangement, there is room given for that part of the head, 
which is above the eye, not to interfere much with the rays 
that go from the object to the mirror ; the aperture of the 
speculum being 4 feet, while the diameter of the tube is 4 feet 
10 inches ; especially as we suppose a night observer will pre- 
fer some kind of warm cap to a hat, the rim of which might 
obstruct a few of the entering rays. 
A long coarse screw-bar is confined in a collet, which takes 
on and off", and may readily be put to the inclosing right' side 
of the seat, so as to present the observer with a short handle. 
The other end of this bar passes into a nut, which, like the 
collet, moves upon a double swivel, so as to admit of every 
motion. The nut is planted upon a machine which will be 
described hereafter, and may be drawn up to any altitude* so 
as to bring the nut upon a level with the swivel of the handle. 
Upon turning the handle, the observer will screw himself, the 
seat, and the telescope, from the ladder ; and may thus follow 
3D 2 
