NEW MICROSCOPES—SEIDEL AND WINTER 203 
ing additional illumination for utilization by both specimen and objec- 
tive? This idea would be entirely practical except for the fact that 
such enlargement of the lenses would increase aberration, both spher- 
ical and chromatic, and apparently present-day lenses are now as highly 
corrected as it is possible for human ingenuity and skillful workman- 
ship tomake them. Spherical aberration, caused by the paraxial rays 
coming to a focus at the center of the lens before those rays near the 
principal axis, is corrected by using concave and convex lenses of 
different material and, consequently, of different refractive index. In 
this manner spherical aberration of a convex lens, for instance, can be 
overcome, without its converging action being altered, by adding to 
the optical system a concave lens in which there is an equal and oppo- 
site aberration. Chromatic aberration, occurring when more than 
one wave length of light is used to illuminate the specimen, is due to the 
fact that the shortest waves of the spectrum are refracted most and 
the longest waves least, thus causing the blue-violet waves to come to 
a focus ahead of the red waves and resulting in a series of colored foci 
all along the axis. Now since, as we have said, the shortening of the 
different groups of wave lengths is not in exact proportion to their 
bending and since this circumstance varies according to the substance 
the light rays pass through, it is possible to combine lenses or lens 
systems in such a way that white light may be obtained. For instance, 
a small concave flint-glass prism produces the same amount of dis- 
persion as a large convex crown-glass prism. Thus, if these two prisms 
are placed with their edges opposite, the crown glass will bring together 
the spectrum produced by the flint glass and white light will be the 
result. However, the rays of white light will not extend parallel with 
the original direction but will bend toward the base of the crown glass 
since the mean refraction of the crown glass is greater than that of the 
flint glass. Achromatic objectives, corrected spherically for one color, 
chromatically for two ; semiapochromatic objectives, possessing moder- 
ate refractive indices and very small dispersion, in which a lens of 
fluorite is substituted for one of the glass lenses; apochromatic objec- 
tives, corrected spherically for two colors, chromatically for three; and 
also certain monochromatic lenses for use with light of one wave 
length only are available for overcoming, at least in part, one of the 
conditions which tends to interfere with better resolution. Con- 
densers, also, can be corrected for both spherical and chromatic aber- 
ration and must be achromatic-aplanatic if the light which enters the 
objective is to come only from the specimen, for condensers with spher- 
ical and chromatic aberration are unable to direct their entire cone of 
light upon the specimen. 
In addition to being as highly corrected as possible and possessing a 
large numerical aperture, an objective should also be capable of ade- 
