154 ASTRONOMY IN WANSTEAD.
of two lenses—a form which is still in use, and known as the
Huygenian eyepiece.
To erect this "Aerial Telescope" (as it was termed) a long pole
was required. At the top of this a pulley was fixed, over which a
rope was carried ; to one end of this was attached a small travelling
platform, balanced on the other side by a leaden weight, the rope
being continued round to the lower end of the platform. The rod
bearing the object glass was fitted with a "cup and ball" joint, the
end of which was, when in use, inserted in a slot in the platform,
which was then hauled up to any height required. Then the
observer, holding the eyepiece by a jointed handle, pulled the long
connecting cord of the object glass taut, using a small winder
attached till the telescope was in focus, steadied his arms on a
wooden rest, and was then ready to make observations.
The illustration of Huygens' Telescope here given is a reduced
copy of the plate in "Christiani Hugenii Zulichemii, Opera Varia
1728," p. 268, which is identical with the plate in his "Astroscopia
Compendiaria," 1684. For the loan of the book from the Library
of the Royal Astronomical Society, I am indebted to Prof.
Meldola, F.R.S.
Such a telescope was a marvellous instrument indeed by com-
parison with those in use at that period, but being of so great a length
was somewhat difficult of management; and we cannot but admire
the patience and skill required in its manipulation, and the dis-
coveries thereby accruing to the science.
This instrument was lent for some time to Dr. Derham, at
Upminster, and in his preface to the 1st edition of "Astro-
Theology," 8vo, 1715, he frequently makes mention of Mr. Huygens'
glass, which he had discovered to be "an excellent glass,"1 and
regrets "the want of a long pole to manage it." In the 3rd edition,
1719, he also mentions the return of the Huygens' glass to the
Royal Society, alluding "particularly to the diligence and accuracy
of my very ingenious friend the Rev. Mr. Pound, into whose
hands the Royal Society have put their noble bequest (sic) of
Mr. Huygens, and who is so well accommodated for raising and
using that glass as to have seen (among other considerable things)
the five satellites of Saturn."
Strangely enough, however, no record exists or entry of the
date when the Royal Society lent the telescope to Dr. Pound.
1 Vide page 162, for his Latin inscription, with signature, scratched on the object-glass.