POSITIVE AND NEGATIVE ABERRATION. 
the object-glass, can pass through the latter, so as to arrive at the 
eye-glass. 
Such are then, generally, the means by which the three con¬ 
ditions of distinct vision with the microscope will be fulfilled. The 
second of these conditions, that which involves the complete cor¬ 
rection of the chromatic aberration, will, however, require here 
some further development, since it involves circumstances which 
have demanded the greatest artistic skill on the part of the 
makers. 
17. It has been shown in our Tract upon “Optical Images,” 53 et 
seq ., that the chromatic aberration of lenses is corrected by combin¬ 
ing together two lenses, one of Hint and one of crown glass, which 
have different effects upon the separation of the coloured images, 
the curvatures of their surfaces being so related, the one to the 
other, that the separation which would be produced by either is 
exactly counteracted by an equal separation in a contrary direc¬ 
tion by the other. If the curvatures, however, of the two lenses 
be not so related as to produce this exact compensation, they may 
either give a predominance to the effect of the one or the other, so 
as to produce chromatic aberrations of opposite kinds; the coloured 
images thus produced being ranged in a contrary order. 
When a single convex lens is used, the most refrangible rays 
are brought to a focus, nearer to the lens than the least refran¬ 
gible ; and consequently the violet and blue images are formed 
nearer to the lens than the red and orange. This is called 
POSITIVE CHEOMATIC ABEEEATION. 
If by combining two lenses of flint and crown glass this aberra¬ 
tion be more than compensated, that is, if the blue and violet 
images are not merely brought to coincide with the red and orange 
ones, so as to render the lens achromatic, but made to interchange 
place with them, so that the red and orange shall be nearest to, 
and the blue and violet farthest from the lens, the chromatic 
aberration will be negative. 
The importance of this in the practical construction of the 
microscope will presently appear. 
It must be remembered that the microscope consists of the 
object-glass, the field-glass, and the eye-glass, and that its 
efficiency depends not merely upon the fidelity of the image pro¬ 
duced by the object-glass, but upon that which is seen by the 
observer looking through the eye-glass. This last must be an 
exact reproduction of the object in form and colour. 
Now it is easy to show that if the object-glass be absolutely 
achromatic, the image seen by the observer through the eye-glass 
will not be so; for, in that case, the rays forming the image pro¬ 
duced by the object-glass passing successively through the field- 
15 
