XXXIV 



Mr. Barlow also applied himself to the improvement of achro- 

 matic object-glasses, and in 1827 communicated a paper on that 

 subject to the Royal Society. In the further course of his inquiries 

 he was led to try the effect of substituting, for the flint-glass lens of 

 the usual achromatic combination, a concave lens formed of trans- 

 parent fluid enclosed in a glass capsule having surfaces of appropriate 

 curvature. The idea of employing fluid lenses, it may be observed, 

 was not new : it had occurred to Newton and David Gregory, and 

 had even been practically applied by Dr. Blair of Edinburgh. Mr. 

 Barlow made choice of sulphuret of carbon as the fluid, which, with 

 a refractive power about equal, has a dispersive power more than 

 double that of flint-glass. He demonstrated the practicability of his 

 method by constructing two telescopes on that principle, with a 

 result sufficiently promising to obtain for him the support of the 

 Board of Longitude in the further prosecution of his experiments: 

 The Council of the Royal Society also, sensible of the important 

 scientific bearing of Mr. Barlow's proposal, engaged Mr. Dollond to 

 construct a fluid-lens telescope under Mr. Barlow's superintendence, 

 and submitted the instrument to be practically tested by competent 

 judges in order to decide on the expediency of constructing a telescope 

 of much larger dimensions on the same principle. The trial instru- 

 ment is described by Mr. Barlow in the Philosophical Transactions 

 for 1833 ; and Reports on its performance, by Sir J. Herschel, 

 Mr. Airy, and Captain (now Admiral) Smyth, will be found in the 

 Proceedings for December of the following year. From the trials 

 made, it appeared that Mr. Barlow's principle might be advan- 

 tageously applied to the construction of a great refracting telescope 

 to be employed in the observation of nebulae and for certain other- 

 astronomical purposes ; but the project seems not to have been 

 further proceeded with. 



Besides publishing the memoirs and treatises mentioned above, 

 Mr. Barlow during his active career was a large contributor to the 

 ' Encyclopaedia Metropolitan. ' He retired from the duties of his 

 Professorship in 1847. In May 1823 he was elected into the 

 Royal Society. He was one of the original Fellows of the Astrono- 

 mical Society, and a member of several of the leading Societies in 

 Europe and America. 



