255 
determining the dispersive ratio of glass^ &c. 
and taking these instead of 27*19 and 271-9, the first surface 
to be rendered proportional ought to have been 28-05, and the 
focus 77.7 inches instead of 80 inches. A new tool was made 
for the first surface, but on measurement it turned out to be 
28-4, viz. -35 of an inch tco long. We determined however 
to proceed with these radii, viz. 
28-4 1 , 26-4*1 r,. . u 
y plate. y flint. Focus 77-9 inches. 
26-4 J 264 j 
The glass being accurately ground to these numbers and 
well centred, the result was satisfactory ; the spherical aber¬ 
ration appeared to be very perfectly balanced, although the 
actual amount of the aberration of the plate lens, in conse¬ 
quence of the excess of the first surface, was 1-738 instead 
of 1.730. The focal lengths now also answered to a dis¬ 
persion d= -664 instead of -660, and yet the correction for 
colour appeared perfect. It is clear, therefore, that an error 
to the amount here stated may exist between the computed 
and the practical radii, without producing any sensible detri¬ 
ment to the effect of the instrument. The plate was now 
reversed, carefully adjusted, and the observation repeated. 
The achromatic correction of course was still perfect, and 
the spherical aberration seemed also tolerably well balanced, 
although the actual amount of the aberration of the plate 
was now only 1-62 instead of 1-73. The preference however 
appeared obviously to belong to the first arrangement. 
Experiment 2. 
19. in order to ascertain the effect of a known want of 
achromatic correction, a plate was employed which had been 
