turned his attention to the improvement of the reflecting telescope and
devised a form of that instrument which still goes under his name. And
even after Chester More Hall in 1729, and John Dollond in 1757, had
shown that chromatic aberration could be nearly eliminated by the
combination of a flint-glass lens with one of crown glass, William
Herschel, who began his observations in 1774, devoted his skill entirely
to the making of reflectors, seeing no prospect of much advance in the
power of refractors.

A refracting telescope which has been freed from the effects of
chromatic aberration is called achromatic. The principle upon which its
construction depends is that by combining lenses of different dispersive
power the separation of the spectral colors in the image can be
corrected while the convergence of the rays of light toward a focus is
not destroyed. Flint glass effects a greater dispersion than crown glass
nearly in the ratio of three to two. The chromatic combination consists
of a convex lens of crown backed by a concave, or plano-concave, lens of
flint. When these two lenses are made of focal lengths which are
directly proportional to their dispersions, they give a practically
colorless image at their common focus. The skill of the telescope-maker
and the excellence of his work depend upon the selection of the glasses
to be combined and his manipulation of the curves of the lenses.

[Illustration: ACHROMATIC OBJECT GLASS.

_a_, crown glass; _b_, flint glass.]

Now, the reader may ask, "Since reflectors require no correction for
color dispersion, while that correction is only approximately effected
by the combination of two kinds of lenses and two kinds of glass in a
refractor, why is not the reflector preferable to the refractor?"

The answer is, that the refractor gives more light and better
definition. It is superior in the first respect because a lens transmits
more light than a mirror reflects. Professor Young has remarked that
about eighty-two per cent of the light reaches the eye in a good
refractor, while "in a Newtonian reflector, in average condition, the
percentage seldom exceeds fifty per cent, and more frequently is lower
than higher." The superiority of the refractor in regard to definition
arises from the fact that any distortion at the surface of a mirror
affects the direction of a ray of light three times as much as the same
distortion would do at the surface of a lens. And this applies eq