324 Professor Barlow on the Cmistruction of 



last Part of the Philosophical Transactions, I have given an ac- 

 count of a series of experiments I made, assisted by the practi- 

 cal skill of Messrs W. and T. Gilbert, instrument makers to the 

 Honourable East India Company, on the construction of refract- 

 ing telescopes; in which memoir I have also described a new in- 

 strument for simplifying the determination of the dispersive 

 power of glass, and I am in hopes that I have so far succeeded 

 in removing from the several formulae those terms which involve 

 quantities too refined to be followed out in practice, that no dif- 

 ficulty of calculation can be said to remain in the construction of 

 this instrument : nor is there any practical one which the inge- 

 nuity of our opticians would not overcome, provided glass could 

 be obtained of sufficient size and purity. But here, unfortu- 

 nately, an impediment interposes; and therefore, with a view to 

 avoid an obstacle we have not at present been able to overcome, 

 I turned my attention to the adoption of some fluid to supply 

 the place of the flint lens. The construction of flint object 

 glasses, retaining, however, the flint lens, had been formerly at- 

 tempted by Dr Blair with considerable success, but which, for 

 some reason, was not afterwards pursued *. I was not at first 

 fully aware of the fluid employed by this ingenious philosopher, 

 and moreover it was at least possible that some other might be 

 found equally well, if not better, suited to the purpose. I there- 

 fore determined to begin de novo, and ascertain with my new in- 

 strument, which was easily made applicable to the purpose, the re- 

 fractive index and the dispersive power of every fluid which ap- 

 peared to possess properties likely to answer my intended pur- 

 pose. I had proceeded some way in this inquiry, with several 

 oils, acids. Sec, when I made trial of the sulphuret of carbon, 

 and here I found at once a fluid which appeared to possess 

 every requisite I could desire. Its index being nearly the same 

 as that of the best flint glass, with a dispersive power more than 

 double, perfectly colourless, beautifully transparent, and, although 

 very expansible, possessing the same, or very nearly indeed •(-, 



• It appears from an article published since this was written, that Mr Blair, 

 the son of Dr Blair, is at piesent engaged in pursuing his father's views. 



•|- I have never found any appreciable numerical difference in the refrac- 

 tive index of this fluid between the temperatures of 31* and 84\the fluid being 

 hermetically sealed. 



