THE ELEMENTS OF MICROSCOPY. 49 
The crown prism C produces the deflection shown by the red 
and blue lines, and this dispersion is restored by the flint prism of 
smaller angle F. 
It will thus be seen that many modifications are possible, by 
combining flint and crown prisms of different angles, an instance 
being found in the direct vision spectroscope of which I have one 
here for your inspection. 
The direct vision spectroscope is often used in conjunction 
with the microscope, and is more often called a micro-spectroscope. 
In this instrument the ray is much dispersed but not deflected, 
this being effected by turning the base of a flint prism towards the 
apex of one of crown glass, cementing them together with Canada 
Balsam. They are usually joined in combinations of three or five, 
one flint between two crowns, or two flints alternately with three 
crowns. 
So much, then, for the effect of a triangular piece of glass upon 
a bundle of rays of white light; we must now consider the effect 
of a lens which may be shown to consist of little else than two 
prisms turned base to base, as you will see by the diagram. 
The top half, and also the lower half of the lens, each acting as 
a real prism, and the violet rays being the most refrangible, come 
to a focus nearer the lens than the red-rays. ‘This effect is known 
as chromatic aberration, and has to be eliminated in the process 
of objective construction. I shall have to refer to this diagram 
again in connection with the construction of lenses for photo- 
micrography, but I wish you all to see how these apparently small 
points are mixed up with microscopical manipulation. 
I showed you in my last paper that a picture might be obtained 
upon a screen without the aid of a lens in somewhat a similar 
manner to the diagram now shown, and you will no doubt expect 
that such a picture would be free from the defects produced by 
chromatic aberration. This is so, but such a picture is by no 
means perfect; there are many objections to the use of sucha 
method, and so we are bound to fall back upon lenses of glass as 
the most convenient means of magnification. 
But lenses of glass, besides being afflicted with errors of chrom- 
atism, are likewise affected by the tendency of the peripheral rays 
to focus themselves at a point between that taken by the central 
ones and the lens itself. The diagram will show this error, which 
is called spherical aberration. 
You will be able by this diagram to see how easy it is, compara- 
tively speaking, to construct the very cheap microscopes one sees 
sometimes, for by cutting down the aperture, that is, by stopping 
out the majority of the peripheral rays, leaving only, say, what is 
represented by the three central ones, a very fair focal point is 
obtained. 
