The Sun.
London, England, 1866-05-19 00:00:00

Royal Institution.

At the meeting of the members of this institution
yesterday evening, the Rev. C. Pritchard, President
of the Astronomical Society, delivered a lecture on
" The TelesCope, its Modern Form, and the Diffi-
culties of its Construction." He gave a general
outline of the history of the telescope, commencing
with the discovery in the ruins of Pompeii of a
spherical piece of glass which had been used by a
jeweller to magnify the figures he engraved. The
invention of spectacles followed, after many ages,
the use of that first known lens, and they were com-
mon in the 16th century, but it was not until the
beginning of the 17th that two Dutch spectacle-
makers accidentally discovered that distant objects
seemed nearer when viewed through two lenses
placed at some distance apart ; and one of those
discoverers formed a telescope by placing two such
lenses in a tube. Those original telescopes magni-
fied about three times, and the objects seen through
them appeared inverted. It was after seeing one of
those rude instruments that Galileo invented a truet
telescope by employing a concave lens for an object
glass. His first telescope magnified six times, and
with his subsequent one, which had a magnifying
power of 12, he discovered Jupiter's moons and the
crescent form of Venus, from which he inferred that
the sun must be in the centre of the solar system,
and that the Ptolomaic theory was erroneous and the
Copernican system the true one. From the time of
Galileo little improvement was made in the principle
of the construction for several years, and telescopes
were formed of unmanageable lengths and with
small diameters, to overcome the difficulty arising
from the dispersion of the coloured rays by refrac-
tion in passing through the object glass. Mr. Prit-
chard explained at some length the difficulties thus
caused, which obstructed any important improve-
ment •in telescopes until Dolland, in the
middle of the last century, succeeded in cor-
recting, after much thought and labour, the forma-
tion of coloured images in the focus of the object
glass by using a combination of lenses made of glass
of different densities and forms. The principle on
which this correction was founded was illustrated by
models, by diagrams, and by the electric lamp, and
it was shown that by the means adopted by Dolland
not only were the coloured rays combined to form
white light, but the aberration of the rays was cor-
rected, and they were all concentrated in the focus.
The mechanical arrangements by subsequent in-
ventors to facilitate the use of the telescope, and to
keep it always turned to the same object in the
heavens, were noticed and commented on, and the
instrument in its perfect state—of which one was
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ing to diagrams of nebulae, and to some beautiful
photographs of the noon, which were taken by means
of the powerful reflecting telescopes of Lord Rosso
and of Mr. Nasmyth.

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