278 On the Manufacture of Optical Glass. [1829. 



Inches. 



Light a direct 10-70 ... 1 ... 1 



Light a reflected by glass 5 ... 36-75 . . . 11-80 . - ~ 



1 ... 40-69 . . . 14-46 V ~ 

 4 ... 43-46 . . . 16-50 . . . ^ 

 9 ... 47-31 . . . 19-56 . . . ^ 5 



6 ... 50-31 . . . 22-12 . . . ^ 



7 ... 51-63 . . . 23-29 . . . ^ 

 3 ... 52-69 . . . 24-26 . . . 



8 ... 54-33 . . . 25-80 . . . ^ 



2 ... 54-56 . . . 26-02 . . . ~- 



The first column refers to the glasses below ; the second 

 gives the distance of the measuring flame b\ the third, the 

 preceding numbers squared and reduced to the direct light as 

 unity; and the fourth, consequently, the proportion of the 

 light a reflected by the first surface of each glass. No. 5 was 

 glass consisting of 1 proportional of oxide of lead, ^ a propor- 

 tional of silica, and l proportional boracic acid. No. 1 was 

 composed of 1 oxide of lead, 1 silica, and 1^ boracic acid. 

 No. 4, of 1 oxide of lead, 1J silica, and 1J boracic acid. 

 No. 9 was flint glass; No. 6, 7 and 3, different pieces of 

 crown glass ; and No. 8 and 2, different pieces of plate glass. 

 1, 3, 5, 6 and 7, were natural surfaces ; 2, 4, 8 and 9, polished 

 surfaces. 



The deficiency of light resulting from the increased re- 

 flecting power, though considerable, may easily be compensated 

 for by slightly increasing the area of the plate ; and the power 

 of obtaining plates of any size is professed to be given by the 

 general process : but whether that expedient involves any 

 other objections, it will be for the optician to determine. 



101. In hardness, these glasses differ from each other as 

 much as in any other quality, and indeed more. The borate 

 of lead is very soft; the biborate of lead is harder, and the 

 triborate equal to flint glass in hardness, The silicated borate 

 of lead is softer than flint glass ; but the glass consisting of 1 

 proportional oxide of lead, 1 of silica, and 1^ proportional of 

 boracic acid, is as hard as ordinary flint glass, at the same 



