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VI. Jin account of the preliminary experiments and ultimate construction of a 

 refracting telescope of 7.8 inches aperture, with a fluid concave lens. In a 

 letter addressed to Davies Gilbert, Esq. President of the Royal Society. 

 By Peter Barlow, Esq. F.R.S. Sgc. 



Read December 18, 1828. 



I HAVE great pleasure in forwarding to you the following account of the con- 

 tinuation of my experiments on the construction of refracting telescopes with 

 fluid lenses ; and after the interest you have taken in the experiments, and the 

 recommendation you were pleased to give on the subject to the Board of 

 Longitude, through whose aid I have been enabled to pursue them, I cannot 

 but flatter myself that it will be satisfactory to you to submit this communi- 

 cation to the Royal Society, who have done me the honour of publishing my 

 first proposition on this subject in their Transactions. 



The instrument I intend more particularly to describe in this paper has a 

 clear aperture of 7-8 inches, exceeding, I think, by about an inch the largest 

 refracting telescope in this country. Its tube is 11 feet, which together with 

 the eye-piece makes the whole length 12 feet ; but its effective focus is, on the 

 principle explained in my former paper*, 18 feet. It carries a power of 700 on 

 the closest double stars in South's and Herschel's catalogue ; and the stars 

 are with that power round and defined, although the field is not then so 

 bright as I could desire. 



The telescope is mounted on a revolving stand, which works with consider- 

 able accuracy as an azimuth and altitude instrument, so as greatly to facilitate 

 the direction of the instrument to any star whose right ascension and declination 

 are given, although it may not be distinctly visible to the naked eye. To give 

 steadiness to the stand it has been made substantial and heavy, its weight by 

 estimation being 400 pounds, and that of the telescope 130 pounds ; yet its 



* Pha. Trans. 1828: Art. VII. 

 MDCCCXXIX. F 



