1829.] On the Manufacture of Optical Glass. 275 



platinum and briefly examined, they are sent to Mr. Dollond, 

 who then enters upon the discharge of his particular duties 

 in the Committee, by cutting, examining, and even working 

 them into telescopes. It is not, however, my place to detail 

 this gentleman's exertions (as a member of the Glass Sub-com- 

 mittee) in the cause of science. They will, I trust, appear in 

 due season ; and I hope that the want of perfect success on my 

 part will not long be a cause of delay. 



2. General qualities of the heavy Optical Glasses. 



93. A great variety of glasses have been formed by the use 

 of different proportions of ingredients. They vary importantly 

 from each other, though by no means to the extent of the 

 difference existing between any of them and flint glass. The 

 specific gravity rises very high in borate of lead, consisting of 

 single proportions, i. e. nearly 24 by weight of boracic acid and 

 112 of oxide of lead ; it is often as high as 6*39 or 6'4, being 

 double that of some specimens of flint glass. In silicated borate 

 of lead, which, in addition to the former quantities, contains 

 16 parts, or a proportional of silica, it is about 5*44. As the 

 proportion of oxide of lead diminishes, so also does the specific 

 gravity lessen, and it is in some of the specimens as low as 4*2 ; 

 still permitting by the proportions present such fusibility and 

 other qualities as consist with the process described. The 

 specific gravity of Guinand's heavy flint glass is about 3'616 ; 

 that of a specimen of ordinary flint glass 3*290; that of plate 

 glass 2-5257 ; and that of crown glass 2*5448. 



97. The refractive and dispersive powers of the glasses 

 increase with their specific gravity, as was to be expected. 

 The powers of two of them, namely, borate of lead and silicated 

 borate of lead, consisting always, if not otherwise expressed, 

 of.single proportionals, have been ascertained by Mr. Herschel, 

 and are as follows : 



Sil. Bor. 

 Bor. Lead. Lead. 



Angle of glass prism 29 6' . . 30 26' 



Refractive index for extreme red rays /L6 = 2'0430 . . 1*8521 

 Refractive index for maximum yellow . ^=2-0652 . . 1*8735 

 Refractive index for extreme violet : : . /4 = 2*1223 . . 1-9135 



Dispersive index = -= (K)740 . , 0-0703 



