Forty-feet Refecting Telescope. 409 
comes to take place, a light iron frame with two small wheels, 
or rather rollers, is pinned to the ends of them. This not only 
keeps them together, but also supports them sufficiently as far* 
as they are to come out. 
A slider, upon an adjustable foundation, is planted at the 
mouth of the telescope so as to be directed towards the centre 
of the mirror. It carries a brass tube, into which all the single 
eye-glasses, or micrometers, are made to slide. When they 
are nearly brought to the focus, a milled head under the end 
of the tube turns a bar, the motion of which adjusts them 
completely. 
The focus of the great mirror is directed to its proper place, 
by putting two plates with springs upon the rim that limits 
the aperture of the tube, into two places which are marked. 
Then a cap with a small hole being put into the sliding tube, 
an assistant with a proper handle must screw in or out one or 
other of the adjusting screws at the back of the mirror, till 
the plates upon the aperture in front of the telescope become 
both visible ; for they are contrived so that when the mirror 
is not properly adjusted, either one or both will vanish. At 
the same time these plates, by their situation, serve to inform 
us which of the screws, whether that to the right or that to 
the left, is in fault, by which means the adjustment becomes a 
very easy operation. 
Slough, near Windsor, 
May 18, 1795. 
WM. HERSCHEL, 
