MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
45 
comparative shadow. The uniformity of the two lights, or at least of their 
relation to each other, was established by trials before and after the experi- 
ments with the reflecting surfaces, and each surface was tried two or three 
times, at intervals, and in a mixed manner ; so that no anticipation of the 
result could in any case bias the mind. The following Table shows the results, 
small decimals being neglected : 
Light a direct 
Inches. 
• • 
o 
© 
r-l 
. . 1 ... 
. . . 1 
5 . 
. 36.75 . . 
o 
GO 
r—i 
r-H 
i 
' • ' 11.8 
1 . 
. 40.69 . . 
. . 14.46 . . . 
1 
• * * 14.4 
4 . 
. 43.46 . . 
. . 16.50 . . . 
1 
* * * 16.5 
9 . 
. 47.31 . . 
. . 19.56 . . . 
1 
• ' ’ 19.5 
6 . 
p-H 
CO 
o 
no 
. . 22.12 . . . 
1 
* ' * 22.1 
7 • 
. 51.63 . . 
. . 23.29 . . . 
1 
* * * 23.3 
3 . 
. 52.69 . . 
. . 24.26 . . . 
1 
* ' * 24.2 
8 . 
. 54.33 . . 
. . 25.80 . . . 
1 
' * * 25.8 
2 . 
. 54.56 . . 
. . 26.02 . . . 
1 
• • • 26. 
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 re- 
duced 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 con- 
sisting of 1 proportional of oxide of lead, \ a proportional of silica, and 1^ 
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, 1^ silica, and 1^ 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 reflecting 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 
