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II. On the performance of Fluid Refracting Telescopes , and on the applica- 
bility of this principle of construction to very large instruments. By Peter 
Barlow, Esq. F.R.S. Cor. Mem. Inst, of France , of the Imperial Academy of 
St. Petersburgh, 8$c. 8$c. 
Read December 9, 1830. 
In the Philosophical Transactions for 1 827, a paper of mine was published 
containing an account of a series of experiments which I had carried on with 
Messrs. W. and T. Gilbert on the curvature of object-glasses for telescopes. 
In the course of these experiments, I saw so much the difficulty which opti- 
cians experience in obtaining large pieces of good flint-glass, that I turned my 
attention to supplying this material by a fluid. Having, after several attempts, 
at length found an admirable substitute in sulphuret of carbon, I wrote a 
short account of my intended construction, addressed to His present Majesty, 
at that time Lord High Admiral, and, as such, President of the Board of 
Longitude, soliciting from that Board assistance in carrying forward my ex- 
periments. Having obtained this aid, the result of my first trial was the con- 
struction of an eight-inch fluid telescope, at that time the largest refractor in 
this country. A description of this instrument is given in the Philosophical 
Transactions for 1829, and some objects are pointed out which had been se- 
lected as tests of its performance. 
I have however since had more time and better means of testing the in- 
strument ; first, through the kindness of Mr. Herschel, who pointed out to 
me several objects that he had observed with his new twenty-inch speculum ; 
and secondly, by direct observations on the same objects in Sir James South’s 
new twenty-feet refractor, and in my own telescope. A few of these, which 
serve to mark distinctly the progress I have made, are given below ; but I 
will first state two or three of my own observations, which, I conceive, tend 
also to the same object. 
In the paper last referred to, I have stated my observation on tj Persei, 
MDCCCXXXI. 
C 
