of Achromatic and Reflecting Telescopes. 315 



" If reflecting telescopes, for which I have a partiality, have, 

 in general, the advantage over refractors, with respect to the 

 distinctness of the images, magnifying power, and smaller focal 

 distance, they must always yield to the latter on account of 

 the smaller apertures which these require, the facility with 

 which they may be applied to different instruments, and the 

 immutability of the substance of their lenses, which render 

 comparable observations made at distant epochs, and partly 

 for the convenience of using them, arising from this, that the 

 object-glass preserves always the well-centered position which 

 the artist has given it. This last quality is so much valued 

 by some observers, that they do not hesitate to prefer a mo- 

 derate achromatic telescope to a good Newtonian one." 



We shall now present, in a small table, the relative perfor- 

 mances of achromatic telescopes, with double object-glasses, 

 and Newtonian reflectors, according to Sir W. Herschel, Mr 

 Herschel, and Professor Amici. 



Ratio of diameters when the per- 

 formance is equal. 

 Achromatic Reflector. 



1 . When the reflectors have a small 



mirror, 



2. When the small mirror is not used, 



3. When the reflectors have a small 10 ) Mr 



mirror, - - 7 T 'o J Herschel. 



4. When the reflector has a small ) Prof. Amici. 



mirror, - - 7| to 10 J 



The following table shows the diameter of an achromatic 

 object-glass that would perform as well as Sir William Her- 

 schePs great forty feet telescope, whose mirror was Jour feet 

 in diameter. 



SirW. 

 Herschel. 



