256 Mr. Barlow on the rules and principles for 
ground to 2 64 and 30 inches, so that the arrangement was 
now 
r" =s — 26*41 
r m = — 264* 
flint. 
This would be the proportionate focal lengths for a dis- 
persion 681. Here the focus was of course too long; and 
the blue colour was so strong as to prevent our judging of 
the effect of spherical aberration. 
The plate was reversed, but the focus and colour of course 
remained the same, as did also the general appearance. As 
far as spherical aberration was concerned, that of the flint 
was obviously over-corrected in one case and under-corrected 
in the other ; but the defect was too much buried in the 
colour to enable us to distinguish the difference. 
Experiment 3 . 
20. Messrs. Gilbert having by them a concave flint lens 
of the same glass to the radii 28*4 and 264, this was matched 
with the last plate : the curves were therefore now 
bn 
j 
r = 26*41 r" — 28*41 
. h plate. _ > flint, 
r = 30* j r r'" — 264* J 
which proportions answer to a dispersion “638, about as much 
in defect as the preceding one was in excess. And it was 
found to have the same general defect ; but the colour was 
now of course yellow. 
Whatever the limits of error may be therefore that can be 
admitted with impunity, they must be far less than those in 
the last two experiments. 
