calculations are founded are more uncertain than at ordinary tempe- 

 ratures ; thus confirming the opinion of Laplace, who says, in speaking 

 of these differences, " qu'elles paraissent e"tre dans les limites des 

 petites erreurs dont cette experience, et les clemens du calcul, dont 

 j'ai fait usage, sont encore susceptibles." The author shows by an 

 elaborate calculation, that the influenqe of moisture in the atmosphere 

 on the velocity of sound must at these two temperatures be quite 

 trifling, not amounting in any case to more than a few inches in a 

 second ; and that this element may therefore be safely neglected in 

 the computation. He next proceeds to compare the experiments of 

 the northern navigators with those of Dr. Van Beck and himself, and 

 also those of other observers, adverting to their general agreement ; 

 which, he observes, is sufficiently satisfactory to warrant the conclu- 

 sion, that whatever difference may still be found to exist between 

 computation and observation, is rather to be ascribed to some imper- 

 fection in the theoretical formula, than to any fault or negligence in 

 the observers. He concludes by remarking the strong testimony 

 which Captain Parry's observations afford of his exemplary accuracy, 

 amidst the great difficulties with which, from the circumstances in 

 which he was placed, he had generally to contend. 



An Account of a Series of Experiments made with a view to the Con- 

 struction of an achromatic Telescope with a fluid concave Lens, in 

 stead of the usual Lens of Flint Glass. In a Letter addressed to Da- 

 vies Gilbert, Esq. M.P. President of the Royal Society. By Peter 

 Barlow, Esq. F.R.S. $c. Read January 17, 1828. [Phil. Trans. 

 1828, p. 105.] 



The idea of constructing achromatic telescopes with fluid lenses 

 was first suggested to the author by the attempt of Messrs. Gilbert 

 to apply to practice the principles and rules for the construction of 

 aplanatic object-glasses, laid down by Mr. Herschel in the Philoso- 

 phical Transactions for 1821. In following these suggestions, the 

 author became sensible of the difficulty of obtaining flint glass of 

 sufficient size and purity for astronomical telescopes ; and was thence 

 led to consider the possibility of substituting some fluid in place of 

 flint glass. Dr. Blair had, many years ago, succeeded in making very 

 perfect telescopes of this description, but he still retained the use of 

 flint glass. Among the various fluids adapted to this optical purpose, 

 the author gave a decided preference to the sulphuret of carbon, 

 which combines properties of perfect transparency and freedom from 

 colour, with a refractive index nearly equal to that of flint glass, and 

 with a dispersive power more than double, properties which it appears 

 to retain under all the temperatures to which it is likely to be ex- 

 posed in an astronomical telescope. After several trials, Mr. Barlow 

 determined the best method of confining this fluid, but was at first 

 unsuccessful in his attempts to construct with it a telescope of 6 

 inches aperture and 7 feet in length. He afterwards undertook a 

 smaller one of 3 inches aperture, which he at length accomplished, 

 and in the very first trials with it was able to separate a great num- 

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