162 ASTRONOMY IN WANSTEAD.
and the apparatus whilst he was using it, viz., a curious micrometer contrived and
made by Mr. Graham, a new eye-glass, a new director to the sight, and a new tin
tube to carry the object-glass."
The various parts of this renowned telescope are still preserved in
the Royal Society's instrument room, where I have had an oppor-
tunity of inspecting them. In their catalogue it is thus described :
No. 22. Huygens' Aerial Telescope.
Having carefully examined and measured this object-glass, I am
able to give the following notes :
It is 71/2 inches in diameter, and on the side, scratched with a diamond point in
his own handwriting, is the maker's name and date, "4 Jany. 1686, C. Huygens"
and further round, in the writing of Dr. Derham, is the inscription, "Vitrum
praestantissimum, W. Derham.''
This glass has been mounted in wood and placed in a case by
Dollond in 1856, under the direction of Dr. Warren De la Rue,
F.R.S.
They also possess two other object-glasses by Huygens, mounted
in a similar manner :
No. 23.—170 ft. focal length. Presented by Sir Isaac. Newton, P.R.S.
Inscription, "C. Huygens 2b Jan, 1686." This is of 8 inches diameter.
No. 24.—An object-glass, with two eye-glasses by Scarlet, for a telescope of
210 ft. Presented by the Rev. Gilbert Burnet, M.A., F.R.S., in 1724. Inscription,
"C. Huygens, 23 Jul, 1686."
The latter is interesting, for we find in a paper by Dr. Bradley
that on the 27th December, 1722, he measured the diameter of
Venus with a Huygenian telescope of 2121/4 ft. This must have been
the object-glass he used in making the measurements.
Although these long refracting telescopes have, for so extended a
period, been superseded by reflectors, it is interesting to know that
those in the possession of the Royal Society were some years ago
(1856 ?) erected on a special stage or tower near the observatory at
Kew, and that Dr. De la Rue then found that the figure (definition)
was absolutely perfect.
This telescope and the other Huygenian object-glasses were