781,[312]
was due. For by the examination with strong lenses of an object which,
even with a power of 227, presented a suspicious appearance, he was able
at once to pronounce its disc to be real, not merely "spurious," and so
to distinguish it unerringly from the crowd of stars amidst which it was
moving.

While the reflecting telescope was astonishing the world by its rapid
development in the hands of Herschel, its unpretending rival was slowly
making its way towards the position which the future had in store for
it. The great obstacle which long stood in the way of the improvement of
refractors was the defect known as "chromatic aberration." This is due
to no other cause than that which produces the rainbow and the
spectrum--the separation, or "dispersion" in their passage through a
refracting medium, of the variously coloured rays composing a beam of
white light. In an ordinary lens there is no common point of
concentration; each colour has its own separate focus; and the resulting
image, formed by the superposition of as many images as there are hues
in the spectrum, is indefinitely terminated with a tinted border,
eminently baffling to exactness of observation.

The extravagantly long telescopes of the seventeenth century were
designed to _avoid_ this evil (as well as another source of indistinct
vision in the spherical shape of lenses); but no attempt to _remedy_ it
was made until an Essex gentleman succeeded, in 1733, in so combining
lenses of flint and crown glass as to produce refraction without
colour.[313] Mr. Chester More Hall was, however, equally indifferent to
fame and profit, and took no pains to make his invention public. The
_effective_ discovery of the achromatic telescope was, accordingly,
reserved for John Dollond, whose method of correcting at the same time
chromatic and spherical aberration was laid before the Royal Society in
1758. Modern astronomy may be said to have been thereby rendered
possible. Refractors have always been found better suited than
reflectors to the ordinary work of observatories. They are, so to speak,
of a more robust, as well as of a more plastic nature. They suffer less
from vicissitudes of temperature and climate. They retain their
efficiency with fewer precautions and under more trying circumstances.
Above all, they co-operate more readily with mechanical appliances, and
lend themselves with far greater facility to purposes of exact
measurement.

A practical difficul