the image. The same eyepieces will serve for either the
reflector or the refractor. But in order that the magnification may be
effective, and serve to reveal what could not be seen without it, the
image itself must be as nearly perfect as possible; this requires that
every ray of light that forms the image shall be brought to a point in
the image precisely corresponding to that from which it emanates in the
real object. In reflectors this is effected by giving a parabolic form
to the concave surface of the mirror. In refractors there is a twofold
difficulty to be overcome. In the first place, a lens with spherical
surfaces does not bend all the rays that pass through it to a focus at
precisely the same distance. The rays that pass near the outer edge of
the lens have a shorter focus than that of the rays which pass near the
center of the lens; this is called spherical aberration. A similar
phenomenon occurs with a concave mirror whose surface is spherical. In
that case, as we have seen, the difficulty is overcome by giving the
mirror a parabolic instead of a spherical form. In an analogous way the
spherical aberration of a lens can be corrected by altering its curves,
but the second difficulty that arises with a lens is not so easily
disposed of: this is what is called chromatic aberration. It is due to
the fact that the rays belonging to different parts of the spectrum
have different degrees of refrangibility, or, in other words, that they
come to a focus at different distances from the lens; and this is
independent of the form of the lens. The blue rays come to a focus
first, then the yellow, and finally the red. It results from this
scattering of the spectral rays along the axis of the lens that there is
no single and exact focus where all meet, and that the image of a star,
for instance, formed by an ordinary lens, even if the spherical
aberration has been corrected, appears blurred and discolored. There is
no such difficulty with a mirror, because there is in that case no
refraction of the light, and consequently no splitting up of the
elements of the spectrum.

In order to get around the obstacle formed by chromatic aberration it is
necessary to make the object glass of a refractor consist of two lenses,
each composed of a different kind of glass. One of the most interesting
facts in the history of the telescope is that Sir Isaac Newton could see
no hope that chromatic aberration would be overcome, and accordingly