166 ON THK MANUFACTURE OF GLASS FOR OPTICAL INSTRUMENTS, 



tude for the improvement of glass for optical purposes, and which com- 

 mittee appointed a sub- committee, consisting of Sir John Herschel, Mr. 

 Dollond, and Dr. Faraday, who in the year 1834 reported progress, that 

 they had succeeeded in making glass plates seven inches square, and 

 eight tenths of an inch thick, tolerably free from bubbles and striae, 

 offers the subjoined questions for the consideration of men of science. 

 Their glass (we repeat particulars for the sake of reference,) was a si- 

 licated borate of lead, composed of 104 parts of nitrate of lead, 24 

 parts of silicated lead, and 42 parts of borax; specific gravity 5.44, 

 refractive index 1.8735, dispersive index 0.0703 ; and was not free from 

 colour. This result, adds Mr. Bourne, does not appear to have been 

 very satisfactory, and I have not heard of any further experiments or 

 results. 



Mr. B. then submits to the attention of men of science the following 

 inquiries ; the replies to which we shall be happy to insert in a future 

 number should any of our correspondents think them worthy of their 

 consideration : 



1 . If we add a small quantity of lead to the materials of crown glass, 

 so as to answer the purpose of a common crown glass lens of an object 

 glass, and also add a larger portion of lead to the same materials, so as 

 to answer the purpose of the common flint glass lens, will not these 

 two kinds of glass have the same character, and produce spectra in 

 which the several colours will be proportional, each to each ? 



2. If we add a very small quantity of lead to the other materials of 

 flint glass, so as to answer the purpose of the crown glass lens, and also 

 add a larger portion of lead to the same materials, so as to answer the 

 purpose of the common flint glass lens, will not these two kinds of flint 

 glass have the same character, and produce spectra in which the several 

 colours will be proportional, each to each ? 



3. Can we use bismuth, or some metal other than lead, in the manu- 

 facture of transparent and colourless glass ?* 



4. As the inflexion of light by angular projections produces nearly 

 the same dispersion that refraction does, and as the best of our polish- 

 ing probably leaves the surface of glass rough and uneven, which would 

 be obvious if we could see the ultimate atoms, may not a considerable 



* We should presume that oxide of bismuth would give a yellow colour to glass, 

 and it is quite doubtful whether we could impart any portion of the fluidity which 

 belongs to the alloys of metallic bismuth to the compounds of its oxide with alka- 

 line and earthy bases. Editors of Amer. Journ. Sc. 8f Arts. 



