THE MICROSCOPE. 
CHAPTER II. 
Mutual chromatic and spherical correction of the lenses (Continued).— 
18. Centering.— 19. Compound object-pieces.— 20. The eye-piece. 
—21. Various magnifying powers adapted to the same microscope. 
—22. Actual dimensions of the field of view.— 23. Means of moving 
and illuminating the object.— 24, Focussing.— 25. Preparation of the 
object.— 26. General description of the structure of a microscope.— 
27. The stage.— 28. The illuminators. —29. The diaphragms. 
Mow let us suppose that the entire combination of lenses is so 
formed, that the negative chromatic aberration produced by the 
object-glass shall be exactly equal to the positive chromatic 
aberration, produced by the field-glass and the eye-glass. In 
that case, it is evident that the one aberration will extinguish the 
other, and that the image viewed by the observer through the eye¬ 
glass will be an exact reproduction of the object, being exempt 
from all aberration whatever. 
To make this more evident, Let ll, fig. 10, be the compound 
object-glass, consisting of a double convex lens of crown glass, 
and a plano-concave lens of flint-glass, formed so as to produce 
negative chromatic aberration; let e F be the field-glass, E E the 
eye-glass, and o the object. 
Let v v u k be the coloured images of the objects, which would 
be produced by l l, if E e were not interposed ; these images will 
be slightly concave towards L L, according to what has been 
explained in our Tract upon ‘ ‘ Optical Images,” 47 et seq .; and since 
l l is supposed to have negative aberration, the red images n e 
will be nearest to it, and the violet ones, v v, most remote from it. 
But the rays which would converge upon the various points of 
these images being intercepted by the field-glass e e, are rendered 
more convergent by it, and the images are accordingly formed 
nearer to it. This lens, E e, also increases the convergence of the 
violet rays which are most refrangible, more than it increases that of 
the red rays which are least refrangible. The consequence of this 
is, that the violet and red images are brought closer together than 
they were, as shown in the figure; but still the violet images are 
more distant from E E than the red, so that the chromatic aberra¬ 
tion of l L and E E conjointly is still negative, though less than 
the aberration of L L alone. 
There is another effect produced by the lens e f which it is 
important to notice. The images produced by l l, which were 
slightly concave towards E F, are changed in their form, so as to 
be slightly concave towards e e. The cause of this change has 
been already explained in our Tract upon “ Optical Images,” 46. 
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