as ^ MR. BARLOW'S ACCOUNT OF THE CONSTRUCTION OF 



I however directed my attention principally to the destruction of the se- 

 condaiy spectrum; and with this view I ordered two 4i-inch tubes, 3 feet 

 Ions', to be fitted up to receive in succession lenses of different focal powers, 

 depending principally upon the value given to n, which I assumed as follows : 

 viz. n = .60, n = .55, n = .50, n = .45, n = .40, the length in each case 

 being 60 inches. Resting on these numbers, the following values were deter- 

 mined, the plate glass having an index .515, the fluid .634, and the dispersive 

 ratio .308. 



Tabular value of the different quantities. 



n = .60 f=Z9.72 /' = 46.42, /" = 48.97 /'" = «1.6 



« = .55 /= 35.53 /' = 34.67, /" = 44.11 /'" = 80.2 



n = .50 / = 33.30 /' = 27.02, /" = 43.35 /'" = 86.7 



n = .45 /== 30.30 /'= 19.91, /" = 43.20 /'" = 96.0 



n =r .40 /= 25.62 /' = 13.30, /" = 44.56 /'" = 111.4 



I soon found, however, that it was impossible to get all the lenses of equally 

 good material and figure ; and as, in consequence, one defect^might be mistaken 

 for another, I altered my plan, and availed myself of the two telescopes I had 

 constructed before, in one of which n = .50, and in the other n = .54. These 

 two I had fitted with other lenses carefully made, making in one the value of 

 n = .60, and in the other n = .40. I had also a new one made with the value 

 of n = .47 ; and after a careful and patient examination of all these five, I 

 determined, and I w£is supported in that determination by othei-s, that the best 

 effect was produced, at least as regarded the object I had in view, when the 

 distance of the lenses was about one half the focal length of the plate lens, 

 and with these proportions, therefore, I determined to construct my 8-inch 

 telescope. 



Construction of the Telescope. 



Having as above stated decided that the distance of the lenses ought to be 

 about half or a little more than half the focal length of the plate lens, I 

 determined upon a focal length of 78 inches for my plate lens, and 59.8 inches 

 for that of my fluid ; which at the distance of 40 inches would produce a focal 

 length of 104 inches, a total length of 12 feet, and an equivalent focus of 18 



