258 
Mr . Barlow ora the rules and principles for 
Experiment 5. 
22. Here the curves of a flint lens were 22-2 and 56 4, both 
concave; the flint index 1-613; that of the plate to be 
matched with it 1*515 ; and the dispersion d = -63 7. 
By the formula we found r = 12-32, and r' = 27-3; but 
the tools actually employed were 12-3 and 27-7, and with 
these the effect was every thing that could be desired ; the 
colour and spherical aberration being both perfectly corrected. 
23. This was an object-glass which had been computed on 
the principles of Mr. Herschel. The index of the flint was 
1*587, of plate 1-515, dispersion -6775, and the focal length 
29-5 inches. 
The radii and foci, as determined by Mr. Herschel’s rule, 
In order to compare this rule with the preceding, I assumed 
the flint radii as above, and computed the radii of the plate. 
Here a = -515, = -587, d = -6775 / = 9-52 
/' = 14-06/" * = 29.5 r" — — 6 66, r" = 34-49 
Experiment 6. 
were r = 19 - 9n r » 
r' = 6-50 
6-66| / = 9*52^ 
4'49J /'=14*06J 
q =- = — 5*17 
r 
f" 
C == “ 4*43 
r 
Whence our equation 
