248 Mr. J. F. W. Herschel on the aberrations of 
tion. There is another case in which an aplanatic eye-glass 
should be employed, viz. in examining the parabolic figure of 
a speculum, or the perfect adjustment of an object-glass. If 
the surface of the speculum or object-glass be divided into 
concentric annuli by diaphragms, covering different parts of 
it in succession, the rays incident on these, after crossing in 
their focus will be spread over corresponding annuli on the 
surface of the eye-glass, and if the distance between the mir- 
ror and eye-glass when adjusted to perfect vision, continue 
the same for all the annuli, we conclude that the figure of the 
speculum is perfect. It is so however only with respect to 
that particular eye-glass ; and if the aberrations of this be not 
corrected, all the pains of the artist will only produce a mirror 
affected with proportional and opposite imperfections. It is 
true, the use of a very high magnifying power obviates this 
objection in great measure, by confining the aberration of the 
eye-glass within a narrower compass ; but it is better in 
theory, and undoubtedly more convenient in practice, to an- 
nihilate it altogether. The aberration in the eye appears to 
me to be entirely out of the question here, but the considera- 
tion of that point would lead us away from the present sub- 
ject. 
On the other hand, when a moderately distinct, but exten- 
sive field of view is of more consequence than a perfect, but 
confined one, as in spectacles, reading glasses, magnifiers of 
moderate power, and eye-glasses for certain astronomical 
purposes, the correction of the aberration in the centre of the 
field, may be sacrificed with little inconvenience. By far the 
best periscopic combination I am acquainted with, consists of 
a double convex lens of the best form, but placed in its worst 
