a Refracting Telescope with a fluid concave lens 331 



focal length of 104 inches, a total length of 12 feet, and an 

 equivalent focus of 18 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 formu- 

 la given in my paper, Phil. Trans. 1827 : Art. XV. I found 

 the proper curves to be 56.4 and 1 44 ; 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 8 

 feet 8 inches, making in all 11 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 ac- 

 curately 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 at- 

 tached 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 and 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 coup- 

 ling 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 backwards 

 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 



