THE MICROSCOPE. 
in fine, then, the rays diverging from the images E' E' v' v', 
after passing through the eye-glass E E, have their divergence 
diminished, so as to diverge from more distant points, 11. The 
divergence of the violet rays,v' v', being most refrangible, is most 
diminished, and that of the red rays, e' e', being least refrangible, 
is least diminished. If their divergence were equally diminished, 
a series of coloured images would be formed at 11, the violet being 
nearer to, and the red farther from e e ; but the divergence of the 
violet, which is already greater than the red, is just so much 
greater than the latter, that the difference of the effects of E E 
upon it is such as to bring the images together at 11. 
Thus it appears, that the positive aberration of the eye-glass 
E E is exactly equal to the negative aberration of L L and e e taken 
conjointly, so that the one exactly neutralises the other, all the 
coloured images coalescing at 11, and producing an image alto¬ 
gether exempt from chromatic aberration. 
There is another important effect produced by the eye-glass ; 
the images e' e' v' v', which are slightly concave towards E e, 
are rendered straight and flat at 11; the principle upon which 
this change depends has been also explained in our Tract upon 
“ Optical Images,” 46. 
Thus, it appears that, by this masterly combination, a multi¬ 
plicity of defects, chromatic, spherical, and distortive, are made, 
so to speak, to extinguish each other, and to give a result, 
practically speaking, exempt from all optical imperfection. 
18. There is still another source of inaccuracy which, though it 
is more mechanical than optical, demands a passing notice. All 
the lenses composing the microscope require to be set in their 
respective tubes, so that their several axes shall be directed in 
the same straight line with the greatest mathematical precision. 
This is what is called centeeing the lenses, and it is a process, 
in the case of microscopes, which demands the most masterly skill 
on the part of the workman. The slightest deviation from true 
centering would cause the images produced by the different lenses 
to be laterally displaced, one being thrown more or less to the 
right and the other to the left, or one upwards and the other 
downwards; and even though the aberrations should be perfectly 
effaced, the superposition of such displaced images would effec¬ 
tually destroy the efficiency of the instrument. 
19. In what precedes, we have, to simplify the explanation, 
supposed the object-glass to consist of a single achromatic lens, a 
circumstance which never takes place except when very low 
powers are sufficient. A single lens, having a very high magni¬ 
fying power, would have so short a focus and such great curvature, 
that it would be attended with great spherical aberration, inde- 
20 
