ABERRATIONS EFFACED. 
To appreciate the circumstances in which these great difficulties 
have consisted, it will be necessary that the reader should revert 
to our Tract upon “Optical Images,” 39 et seq. It is there shown, 
that when an object is placed before a convex lens, the image of it 
which is produced, is not in any case a faithful copy of the object. 
In the first place, each portion of the lens, proceeding from its 
centre to its borders, produces a separate image; this series of 
images, being ranged at different distances from the lens: when 
these images are looked at, as they would be, for example, with 
the eye-glass of the microscope, they are seen projected one upon 
another, and being slightly different in their magnitudes, a con¬ 
fusion of outline and lineaments ensues, so that the object appears 
as though it were viewed through a mist. 
This sort of indistinctness, called spherical aberration , has been 
fully explained in our Tract upon “ Optical Images,” and the 
general principles, by which its effects may be more or less miti¬ 
gated, have been there explained. 
It has been in the diminution, if not entire extinction, of this 
cause of indistinctness, by the happy adaptation of the curvatures 
of the lenticular surfaces entering into the optical combinations 
which form the microscope, that the address and genius of the 
practical opticians has been chiefly manifested ; and if it cannot 
be stated, with strict truth, that all the effects of spherical aber¬ 
ration have been effaced in the best instruments now placed at 
the disposition of the observer, it may, at all events, be safely 
affirmed, that they exist in so small a degree as to offer no serious 
impediment to his researches. 
But independently of this source of indistinctness, there is 
another which has also been fully explained in our Tract upon 
“ Optical Images,” 39. 
Light is a compound principle, consisting of several elements, 
differing in colour and also in refrangibility, the consequence of 
wffiich is, that when an object is placed before a convex lens, it 
is not one image which is formed of it, but a series of images, 
varying in colour, from a violet or blue, through all the tints of 
the rainbow, to a red; these images are placed at slightly dif¬ 
ferent distances from the lens, and when viewed through the eye¬ 
glass, would be projected one upon the other, and being of slightly 
different magnitudes, the consequence of such projection would 
be, that their outlines, and those of all their parts, would be 
more or less fringed with iridescent colours, an effect which, 
it is needless to say, would, destroy the distinctness of the 
lineaments. 
12. The principle upon which this chromatic aberration is 
counteracted, has been fully explained in our Tract upon “Optical 
9 
