great mass, was
always behindhand with the atmosphere in thermometric changes, which was
very injurious to the distinctness of the images.

Herschel found that in England, there are not above a hundred hours in a
year during which the heavens can be advantageously observed with a
telescope of forty feet, furnished with a magnifying power of a
thousand. This remark led the celebrated astronomer to the conclusion,
that, to take a complete survey of the heavens with his large
instrument, though each successive field should remain only for an
instant under inspection, would not require less than eight hundred
years.

Herschel explains in a very natural way the rare occurrence of the
circumstances in which it is possible to make good use of a telescope of
forty feet, and of very large aperture.

A telescope does not magnify real objects only, but magnifies also the
apparent irregularities arising from atmospheric refractions; now, all
other things being equal, these irregularities of refraction must be so
much the stronger, so much the more frequent, as the stratum of air is
thicker through which the rays have passed to go and form the image.

Astronomers experienced extreme surprise, when in 1782, they learned
that Herschel had applied linear magnifying powers of a thousand, of
twelve hundred, of two thousand two hundred, of two thousand six
hundred, and even of six thousand times, to a reflecting telescope of
seven feet in length. The Royal Society of London experienced this
surprise, and officially requested Herschel to give publicity to the
means he had adopted for ascertaining such amounts of magnifying power
in his telescopes. Such was the object of a memoir that he inserted in
vol. lxxii. of the _Philosophical Transactions_; and it dissipated all
doubts. No one will be surprised that magnifying powers, which it would
seem ought to have shown the Lunar mountains, as the chain of Mont Blanc
is seen from Macon, from Lyons, and even from Geneva, were not easily
believed in. They did not know that Herschel had never used magnifying
powers of three thousand, and six thousand times, except in observing
brilliant stars; they had not remembered that light reflected by
planetary bodies, is too feeble to continue distinct under the same
degree of magnifying power as the actual light of the fixed stars does.

Opticians had given up, more from theory than from careful experiments,
attempting high magnifying powers, even for