258 
Mr. Barlow on 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 I'Sis ; that of the plate to be 
matched with it 1*515 ; and the dispersion d = *637. 
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. 
Experiment 6. 
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, 
r' — 6*50/ r'" = -}- 34 ' 49 J f = 14*06J 
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, =*6775 f = 9-52 
/' = 14*06/" * = 29.5 r" = — 6*66, r" = 34*49 
c’ 
c 
a'f' — r'" ^ 
_ £1 _ _ 
43 
Whence our equation 
