192 
MR. LISTER ON THE IMPROVEMENT OF 
central rays, and the colours of the violet side of the spectrum into which each 
ray is refracted have also their foci shorter than those of the red side ; if 
either of these errors is but partially removed by the concave lens, the glass is 
said to be under-corrected as to that aberration, and over-corrected if the op- 
posite error is produced by it. 
A large focal pencil free from all aberration is evidently the great requisite 
for the object-glass of the compound microscope ; a second point desirable to 
be attained is, that the field should be flat and well defined throughout; and a 
third, that the light admitted should as much as possible be only such as goes 
to form the picture, and should not be intercepted or diffused over the field by 
too many reflections. 
The prominent obstacle to obtaining a sufficient pencil for high powers by 
one object-glass of large aperture and deep curves, is that the correction for 
the spherical figure by the concave lens is greater for the rays of the circum- 
ference than its due proportion to that for the more central ones; so that when 
such a glass is corrected for the mean of the pencil, if we suppose its disk 
divided into a central space and three rings surrounding it, the rays which 
pass through the central space and those of the second ring from it will arrive 
at their focus when those of the first ring will have just crossed the axis, and 
those of the marginal ring will not quite have reached it. The injury resulting 
to the defining power is in similar glasses inversely as the squares of their focal 
lengths, as far as regards this cause of error, as well as those which arise from 
incorrigible colour and defects of workmanship. The effects upon the pencil 
which have been before described, must not be included under the same law. 
This excess of correction in the marginal rays increases after a certain point 
mi rapidly with a small enlargement of the aperture of the glass, as soon to 
prescribe a limit, beyond which it cannot be carried without injury to the 
picture. 
With glasses of more contracted aperture and at the several surfaces of 
which the marginal refraction is moderate, the effect alluded to is compara- 
tively inconsiderable; and consequently by dividing the refraction among two 
or more such glasses corrected for the rays that pass through them, the pencil 
received may be enlarged without impediment, and the light and distinctness 
greatly increased, thus constituting the important advantage of combination. 
