36 
MR. BARLOW’S ACCOUNT OF THE CONSTRUCTION OF 
I however directed my attention principally to the destruction of the se- 
condary spectrum ; and with this view I ordered two 4^-inch tubes, 5 feet 
long-, 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 = 39.72 
/' = 46.42, 
f" = 
48.97 
/" = 
81.6 
n = 
.55 
/= 35.53 
/' = 34.67, 
f = 
44.11 
/"' = 
80.2 
n = 
.50 
/ = 33.30 
f = 27.02, 
f' = 
43.35 
/'" = 
86.7 
n = 
.45 
/ = 30.30 
/' = 19.91, 
/" = 
43.20 
f" = 
96.0 
n = 
.40 
f— 25.62 
f - 13.30, 
f' = 
44.56 
f" = 
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 defecffmight 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 was supported in that determination by others, 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 
