t62 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 tin's 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, 



(i. — An object-glass of 122 ft. focal length, with an eye-glass of 6 

 I inches, and original apparatus for adjustment, made b}- Huygens, 



I and presented b}' him to the Royal Society in 1691. 



12 part?. \ 2. — The apparatus for using Huygens' object-glass, constructed by 

 j Hooke. 



3, — Additional apparatus by Dr. Pound, presented by Dr. Bradley. 

 1^4. — Ditto by Mr. Cavendish. 



Having carefully examined and measured this object-glass, I am 

 able to give the following notes : 



It is 7i 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, "\'itrum 

 prirstantissimum, W. Derham." 



This glass has been mounted in wood and placed in a case by 

 DoUond in 1856, under the direction of Dr. AVarren 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. Hu3'gens 26 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 

 A'enus with a Huygenian telescope of 212} 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 period, been superseded by reflectors, it is interesting to know that 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 



