THEORY OF THE MICROSCOPE. 
chromatic dispersion is small near the centre of the lens, and 
like spherical aberration, increases very rapidly towards the 
borders of the lens. 
In this figure, as in Fig. 2, F f represents the longitudinal 
spherical aberration, while the chromatic aberration extends 
over the entire space from F to F. The chromatic aberration 
is therefore in the same direction as the spherical aberration. 
Here the narrowest part of the pencil, and hence the most 
available focus, is not at a 5, but at s s. In this case, as with 
spherical aberration, if the lens were placed with its convex 
surface toward the parallel rays, it is obvious that the longitu- 
dinal chromatic aberration F f would be diminished, but the 
actual dispersion of the colors in each ray of light would 
remain unchanged, except as the change of position causes a 
slight alteration of the refractive power of the lens. 
11. The first efforts to diiiiiiiisli the Chromatic Aber- 
ration of a single lens consisted in reducing its diameter, as is 
seen in the cheap microscopes so commonly found in the 
market : in these the orifice through which the light passes is 
exceedingly small, and consequently the object appears but 
feebly illuminated. In the compound microscope of such con- 
struction only a low magnifying power can be used. 
12. The second method of diminishing the Chromatic 
Aberration consists in employing two or three lenses of shal- 
low curvature placed close together, by which means the chro- 
matic and spherical aberrations are made as small as is possible 
with lenses composed of a single kind of glass. This form of 
lenses, called doublets and triplets, is also seen in cheap com- 
pound microscopes of French and German manufacture. But 
in this form a considerable amount of chromatic aberration still 
remains, even when the diameter of the lenses is quite small. 
13. Achromatism. From the preceding considerations it 
is evident, that it is of primary importance, in the construction 
of a really efficient microscope, that the chromatic and spheri- 
cal aberration should be entirely corrected^ but that no such 
correction can be effected in a single lens. 
Hear the middle of the last century, John Dollond of Lon- 
J. & W. GRUNOW k GO’S ILLUSTRATED 
