I 



330 Mr Barlow's account of the Construction of 



I, however, directed my attention principally to the destruc- 

 tion of the secondary spectrum ; and with this view I ordered 

 two 4^ inch tubes, five feet long, to be fitted up to receive in 

 succession lenses of different focal pov/ers, depending princi- 

 pally upon the value given to «, which I assumed as follows : 

 viz. n — .60, n = .55, n = .50, n = .45, n =r .40, the length 

 in each case being sixty inches. Resting on the^e numbers, 

 the following values were determined, the plate glass having 

 an index .515, the fluid .634, and the dispersive ratio .308. 



Tabular value of the different quantities, 

 n = .60 /= 39.72 

 n = 'So f— 35.53 

 n = .50 /= 33.30 

 n = .45 /= 30.30 

 n = .40 /= 25.62 



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

 lenses of equally good material and figure ; and as, in conse- 

 quence, one defect might be mistaken for another, I altered 

 my plan, and availed myself of the two telescopes I had con- 

 structed 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. T 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 was supported in that determination 

 by others, that the best effect was produced, at least as regard- 

 ed 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 



