A REFRACTING TELESCOPE WITH A FLUID CONCAVE LENS. 37 



feet. For the curves of the parallel meniscus checks for containing 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 sci-ew 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. 



f 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 



