a Large Re/racHng Telescope, 



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In this figure A B C D represent the tube of 

 the 6 inch telescope, C D the plate object glass, 

 V the first focus of rays, d e the fluid concave lens, 

 distant from the former 24 inches. The focal 

 length M F being 48 inches, and, consequently, 

 as 48 : 6 : : 24 : 3 inches, the diameter of the fluid 

 lens. The resulting compound focus is ^'H,-^ 

 inches ; it is obvious, therefore, that the rays df^ 

 efy arrive at the focus under the same convergency, 

 and with the same light as if they proceeded from 

 a lens of 6 inches diameter, placed at a distance 

 beyond the object glass C D, (as C ly), deter- 

 mined by producing these rays till they meet the 

 sides of the tube produced in C ly, viz. at Q^.5 

 inches beyond the fluid lens. Hence, it is obvious, 

 the rays will converge as they would do from an 

 object glass, C D', of the usual kind, with a focus 

 of 10 feet 5 inches. We have thus, therefore, 

 shortened the tube 38*5 inches, or have at least 

 the advantage of a focus 38*5 inches longer than our 

 tube ; and the same principle may be carried much 

 farther, so as to reduce the usual length of refract- 

 ing telescopes nearly one-half, without increasing 

 the aberration in the first glass beyond the least 

 that can possibly belong to a telescope of the usual 

 kind of the whole length. It should, moreover, be 

 observed, that the adjustment for focus may be 

 made either in the usual way, or by a slight 

 movement of the fluid lens, as in the Gregorian 

 reflectors, by means of the small speculum. In the 

 latter case, the eye-piece is fixed, which may pro- 

 bably be convenient for astronomical purposes, in 

 consequence of the great delicacy of the adjustment. 



Thus far every thing is in favour of the pro- 

 posed construction ; but some doubtful points 

 may probably present themselves, viz. Is not the 

 opening of the lenses to so great a distance calcu- 

 lated to produce an irrationality in the two spectra ; 



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