Prof. Barlow's description of a New Fluid Telescope. 95 



In this figure A B C D represent the tube of the 6 inch te- 

 lescope, C D the plate object-glass, F 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 Q%5 inches ; it is obvious, there- 

 fore, that the rays df, ef 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 D''), determined by producing 

 these rays till they meet the sides of the tube produced in C D', 

 viz. at Q%6 inches beyond the fluid lens. Hence, it is obvious, 

 the rays will converge as they would do from an object-glass, 

 CD', 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 far- 

 ther, so as to reduce the usual length of refracting 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 ei- 

 ther in the usual way or by a slight movement of the fluid 

 lens, as in the Gregorian reflectors by means of the small spe- 

 culum. In the latter case the eye-piece is fixed, which may 

 probably be convenient for astronomical purposes, in conse- 

 quence of the great delicacy of the adjustment. 



It is obvious, from what is stated above, that a great range 

 is open to experiment, as to the determination of the best po- 

 sition of the fluid lens. If, as has been stated, the opening of 

 the glasses has of itself a tendency to produce irrationality, 

 and that the degree and quality may be changed by different 

 distances, it is of course desirable to find that distance where 

 the quality and degree of irrationality from this cause may 

 counteract that due to the irrationality of the spectra, and 

 thus produce either a complete rationality, or at least the mi- 

 nimum of irrationality. I believe I am already very near this 

 point ; because, unless this were the case, we could scarcely 

 expect to find the stars so perfectly colourless and well-defined 



