Mr. Barlow on the Refracting Telescope. 9 



All the above variations may be considered as belonging to 

 one class, distinguished by the condition of /' being equal to 

 /"'. Another class belongs to the case of /"' = 0, which is 

 the form I have adopted in my fluid telescope. In this the 

 range is more limited : we cannot here with any advantage 

 make n greater than unity, while we employ flint-glass. With 

 Faraday's glass, we can make n any number between 1 and 4f ; 

 and with sulphuret of carbon, any number between 1 and ^. In 

 the former class the focal power remains throughout the same 

 as the length of the telescope ; but in the latter it may be 

 increased with the sulphuret of carbon to nearly double the 

 length, and with Faraday's glass to l times the length. It is 

 not, however, necessary to confine ourselves to either of these 

 classes: we may make f" = qf' 9 and take (/, any number 

 within practicable limits, greater or less than unity ; and while 

 q is less than unity, the focal power of the telescope will be 

 increased, and with q greater than unity, it will be diminished. 



Hence it appears that the refracting telescope, which has 

 for about eighty years been limited to one particular form, will 

 admit of an immense variety of untried forms, some of which 

 seem to offer important advantages. 



In all these cases, for example, the size of the concave cor- 

 recting lens, which has hitherto set a limit to the dimensions 

 of refracting telescopes, may be considerably diminished, leav- 

 ing the aperture and light the same ; or, which is equivalent 

 with any given correcting lens, we may employ a much greater 

 front lens. 



In one class, also, we can, by increasing the focal power, 

 diminish the length of the telescope ; and in another, by dimi- 

 nishing the focal power, increase the light. 



We have, also, as I have shown in the ( Philosophical 

 Transactions' for 1828, at all events a control over the amount 

 of the secondary spectrum, if not the power of destroying it 

 altogether; and, lastly, the error arising from spherical aber- 

 ration may be diminished almost without limit. 



On the latter subject it may be asked, how (as we destroy 

 or counteract the spherical aberration entirely in the common 

 telescope) can it be farther diminished? To this I can only 

 reply, that however completely we destroy it in our formulae, 



