8 Mr. Barlow on the Refracting Telescope. 



than 2; with Faraday's glass, the focus being nearly double 

 and the refractive power very great, the curves will be of much 

 greater radii, and the principle would admit of some consider- 

 able extension ; but with sulphuret of carbon it may, as has 

 been stated in a former part of this article, be pushed so far as 

 to enable us to make the whole correction for colour in the 

 eye-tube only, that is, in a tube of Jth orith the focal length of 

 the telescope. 



We have seen, for example, still taking the whole focus, 

 7 feet or 84 inches, that when 



n - 2, / = 42 and /' = / s= 49 inches. 



If n = 3, / = 28 and /' = \f = 21 T V inches. 



If n = 4, /= 21 and /' = &/= llf inches. 



If n - 5, /= 16.8 and/' = &/= 7 li inches - 



If n = 6, /= 14 and /' = T 7 8 /= 4f inches. 



If n = 7, /= 12 arid f rr / T /= 4 inches. 



If n == 8, /= 10 and /' = & f = 3-fo inches, 



which latter case even (having regard to the reduced aper- 

 ture) is still as practicable as the flint and crown corrector at 

 half the focal distance. 



With Faraday's glass it has been shown, that with a focus 

 of 84, and with n = 2, we have 



n = 2, /= 42 and /' = ^/= 18.9 inches. 



If n = 3, / = 28 and /' = -, f = 8.4 inches. 



If 7i = 4, /:= 21 and /' = -^ / == 4.7 inches. 



If n =: 5, /== 16.8 and f = ^ 9 /= 3.02 inches, 

 the case n = 4 being perfectly practicable. Thus it appears 

 the greatest extension that can be given to this principle with 

 flint glass is n = 2 ; with Faraday's glass, n = 4, and with 

 sulphuret of carbon, n = 8 : that is, in the first case, a plate 

 lens of any aperture can be corrected with a flint lens of half 

 that aperture ; with Faraday's glass, by a lens of one-fourth 

 the aperture ; and with sulphuret of carbon, with a lens of one- 

 eighth the aperture : and it will be observed that, between the 

 values of n = 1 and n = 2, each of these mediums admit of 

 unrestricted application the case n = 1 in each being the 

 triple object-glass, under that particular arrangement in which 

 the focus of the first lens is equal to the focal length of the 

 telescope. 



