50 THE MICROSCOPICAL NEWS. 
I will now show you another diagram, illustrating chromatic 
aberration, from which it will be seen how curiously the various 
rays intersect each other :— 
It is a difficult thing to entirely separate chromatic aberration 
from spherical errors, in order to study it by itself, but it may be 
done by using mono-chromatic light. 
We may now see a dawning of the processes used for achrom- 
atizing lenses. Certain combinations of crown and flint are put 
together, in order that there may be no colour in the image pro- 
duced by them, and it may be inferred from what has been 
previously shown, that very diverse effects may be produced by 
altering the various curves of the component flint and crown lenses, 
and their relative thicknesses. 
The top figure of the diagram shows a prism in which a ray of 
white light has been decomposed by the crown prism C, and amal- 
gamated again by the small flint prism F, and the application of 
this principle to the construction of objectives is shown in the side 
figure, where a concavo-convex of flint is cemented to a double 
convex of crown. 
A lens in its natural state is very much under-corrected ; the 
violet rays, as I have already shown you, are brought to a focus 
between the lens and the focal point of the red rays—now, it is 
possible to reverse this order of things, and by applying a flint 
concave of sufficient strength, to bring the red rays to a focus at a 
point intermediate between the lens and the focal point of the 
violet. Such a lens is called over-corrected, and is shown dia- 
grammatically by the figure on the screen. This was the condition 
in which all the older lenses were turned out, as it was thought 
necessary to do so in order to correct the aberrations of the 
Huyghenian eye-piece. 
IT now come to a point which will no doubt interest you all, viz.,— 
the true explanation of what is meant by the focus of a lens. 
You will doubtless know that parallel rays, such as those of the 
sun, falling upon a double convex lens of crown glass, bring those 
rays to a focus substantially at the radius of the curvature of that 
lens; but if the lens is plano-convex, the rays will focus themselves 
at the diameter of curvature. 
Now, the sidereal focus of a lens, or the focus of the sun’s rays, 
is always shorter than those obtained in the microscope. If we 
shorten the posterior focus we gradually lengthen the anterior 
focus, so that when the rays, falling on a double convex lens from 
a distance of the diameter of its curvature, are brought to a focus, 
they fall at the same distance on the opposite side of the lens. 
The nearer the source of light to the centre of curvature, the more 
parallel will the outer rays become; the longer the anterior focus, 
the shorter the posterior. 
