CHROMATIC ABERRATION. 
5 
by which it is formed, and when this curved image is viewed 
by another lens, as in the common compound microscope, the 
distortion of the image is still further increased. 
This effect, called aberration of sphericity, or curvature of 
the image, is not diminished by contracting the aperture of the 
lens, nor even by dividing the refraction between two convex 
lenses, so placed as to act together as a single lens ; but when 
the longitudinal spherical aberration is corrected, the aberra- 
tion of sphericity, by proper arrangements, may be corrected 
also, so that every part of a flat object shall be distinctly 
defined upon a flat field. This quality, called flatness of fleld 
is very essential to a good microscope. 
10. Cliroinatic Aberration is another error which arises in 
the use of a single lens. Whatever be the form of the lens, 
the material of which it is composed does not act uniformly 
upon the differently colored rays of which white light is com- 
posed, but separates each ray of white light, falling obliquely 
upon its surface, into the colors of the prismatic spectrum. 
Fig. 3. 
Let L L, Z Z, Fig. 3, represent rays of white light falling upon 
a piano convex lens. The ray Z Z, being nearer the border of the 
lens is strongly refracted, and the blue ray Z B diverges widely 
from the red ray Z R, and generally, the blue rays are brought 
to a focus nearer to the lens than the red rays. 
The rays L L, falling very near the axis of the lens, are 
almost perpendicular to the refracting surface, and hence the 
colored rays of which the white light L L is composed, are but 
slightly separated from each other. From this we see that the 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
