A REFRACTING TELESCOPE WITH A FLUID CONCAVE LENS. 
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feet. For the curves of the parallel meniscus checks for containing 1 the fluid, 
I proposed — 30 inches and + 144 inches, the latter towards the eye, and 
then computing the proper curves for the plate by the formula given in my 
paper, Phil. Trans. 1827: Art. XV. I found the proper curves to be 56.4 and 144, 
and to these curves Messrs W. and T. Gilbert worked the several glasses and 
the circular ring. Mr. Donkin undertook to draw the tubes, which I was desirous 
of having 8 inches in the interior diameter, but his nearest treblet was only 
7.8 inches, to which size therefore I was confined. The tube was drawn in 
three pieces, each 3 feet 8 inches, making in all 1 1 feet ; and to this the pipe for 
the eye-piece being attached, gave the full length 12 feet : two of the above 
pieces of 7.8-inch tube are strongly and accurately jointed by a lining piece, 
and the other part is made to screw on for more conveniently getting in and 
adjusting the fluid lens which is near this joint, and is inclosed in a cell which 
screws on to an interior tube 5 inches in diameter, and 3 feet 6 inches long, 
sliding in two collars properly turned for the purpose, having a notch in each 
to receive a feather attached externally to the tube to preserve a parallel 
motion. 
The other end of this tube of course reaches to within about 4 feet of the eye 
end of the large tube, and to the former is fixed a brass nut properly fitted to 
receive a screw on the end of a brass rod 4 \ feet in length ; this rod works 
in a coupling box or collar, fixed on the inside of the large tube about 1 foot 
9 inches from the end, and the end of the rod passes through the front end of 
the large tube, where it is cut square to receive a milled head or a universal 
joint key, by means of which the tube carrying the cell may be moved back- 
wards or forwards ; and the adjustment is thus made for colour in the first 
instance, and afterwards the focus is obtained by the usual rack motion. 
The difficulty of centering two lenses at so great a distance from each other 
is considerable, if not properly provided for. In this instance the front lens is 
placed in a thin detached cell and confined by a counter cell. It is then 
placed with its first cell in another which screws and unscrews at the object 
end of the telescope as usual ; except that the last cell is sufficiently large to 
admit of adjusting the interior one carrying the lens by means of two pair of 
opposite pushing screws. These provisions being made, the telescope is placed 
opposite to a proper object, the centering is produced by trial, by means of 
