aked eye, he undertook to do
away with all doubt, by direct observations.

On examining some pins' heads placed at a distance in the open air, with
a three-foot telescope, Herschel could easily discern that those bodies
were round, when the subtended angles became, after their enlargement,
2' 19". This is almost exactly the result obtained with the naked eye.

When the globules were darker; when, instead of pins' heads, small
globules of sealing-wax were used, their spherical form did not begin to
be distinctly visible till the moment when the subtended magnified
angles, that is, the moment when the natural angle multiplied by the
magnifying power, amounted to five minutes.

In a subsequent series of experiments, some globules of silver placed
very far from the observer, allowed their globular form to be perceived,
even when the magnified angle remained below two minutes.

Under equality of subtended angle, then, the telescopic vision with
strong magnifying powers showed itself superior to the naked eye vision.
This result is not unimportant.

If we take notice of the magnifying powers used by Herschel in these
laborious researches, powers that often exceeded five hundred times, it
will appear to be established that the telescopes possessed by modern
astronomers, may serve to verify the round form of distant objects, the
form of celestial bodies even when the diameters of those bodies do not
subtend naturally (to the naked eye), angles of above three tenths of a
second: and 500, multiplied by three tenths of a second, give 2' 30".

Refracting telescopes were still ill understood instruments, the result
of chance, devoid of certain theory, when they already served to reveal
brilliant astronomical phenomena. Their theory, in as far as it depended
on geometry and optics, made rapid progress. These two early phases of
the problem leave but little more to be wished for; it is not so with a
third phase, hitherto a good deal neglected, connected with physiology,
and with the action of light on the nervous system. Therefore, we should
search in vain in old treatises on optics and on astronomy, for a strict
and complete discussion on the comparative effect that the size and
intensity of the images, that the magnifying power and the aperture of a
telescope may have, by night and by day, on the visibility of the
faintest stars. This lacuna Herschel tried to fill up in 1799; such was
the aim of the memoir entitled, _On the spa