dition of a museum inmate.

The exceedingly high magnifying powers employed by Herschel constituted
a novelty in optical astronomy, to which he attached great importance.
The work of ordinary observation would, however, be hindered rather than
helped by them. The attempt to increase in this manner the efficacy of
the telescope is speedily checked by atmospheric, to say nothing of
other difficulties. Precisely in the same proportion as an object is
magnified, the disturbances of the medium through which it is seen are
magnified also. Even on the clearest and most tranquil nights, the air
is never for a moment really still. The rays of light traversing it are
continually broken by minute fluctuations of refractive power caused by
changes of temperature and pressure, and the currents which these
engender. With such luminous quiverings and waverings the astronomer has
always more or less to reckon; their absence is simply a question of
degree; if sufficiently magnified, they are at all times capable of
rendering observation impossible.

Thus, such powers as 3,000, 4,000, 5,000, even 6,652,[311] which
Herschel now and again applied to his great telescopes, must, save on
the rarest occasions, prove an impediment rather than an aid to vision.
They were, however, used by him only for special purposes,
experimentally, not systematically, and with the clearest discrimination
of their advantages and drawbacks. It is obvious that perfectly
different ends are subserved by increasing the _aperture_ and by
increasing the _power_ of a telescope. In the one case, a larger
quantity of light is captured and concentrated; in the other, the same
amount is distributed over a wider area. A diminution of brilliancy in
the image accordingly attends, _coeteris paribus_, upon each
augmentation of its apparent size. For this reason, such faint objects
as nebulae are most successfully observed with moderate powers applied to
instruments of a great capacity for light, the details of their
structure actually disappearing when highly magnified. With stellar
groups the reverse is the case. Stars cannot be magnified, simply
because they are too remote to have any sensible dimensions; but the
space between them can. It was thus for the purpose of dividing very
close double stars that Herschel increased to such an unprecedented
extent the magnifying capabilities of his instruments; and to this
improvement incidentally the discovery of Uranus, March 13, 1