MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
43 
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 : — 
Bor. Lend. Sil. Bor. Lead. 
Angle of glass prism 29° 6' . . 30° 26' 
Refractive index for extreme red rays . . . p = 2.0430 . . 1.8521 
maximum yellow . . . — = 2.0652 . . 1.8735 
extreme violet : : . . . — = 2.1223 . . 1.9135 
* ^ 
Dispersive index = ^ = 0.0740 . . 0.0703 
These intense powers upon light are not accompanied by any circumstance 
rendering the glass optically unfit for the compensation of the dispersive powers 
of crown or plate glass. Three object-glasses have been constructed for the 
express purpose of ascertaining this point ; and all of them tend to demonstrate 
that the compensation or correction may be effected with equal if not greater 
facility than with flint glass. 
98. One important circumstance connected with the application of these 
glasses to the purposes for which they are designed, is their colour. The great 
power they have of developing strong tints from metallic impurities, has been 
already described and illustrated (22. 23), and creates a difficulty in the way 
of obtaining them unobjectionably free from colour. The usual colour is more 
or less of yellow, and is perhaps almost altogether, if not quite, dependent 
upon the presence of a little iron. Like many of those dependent upon mineral 
substances, it is very much heightened by elevation, and lessened by diminution 
of temperature. It is rapidly and permanently diminished by increasing the 
proportions either of the silica or the boracic acid. The silicated borate of 
lead has latterly been obtained of such faint tint by the precautions relative 
to impurities, already described, that, when 9 inches in thickness, white paper 
looked at through it in open daylight resembled in appearance and depth of tint 
the surface of a lemon. Glass consisting of 1 proportional = 112 oxide of lead, 
1 proportional = 16 silica, and 1^ proportional = 36 boracic acid, when 7 inches 
in thickness and examined in the same manner, did not give a colour surpassing 
that of pale roll sulphur. The tri-borate of lead glass is almost as colourless as 
good flint glass, but might perhaps be found objectionable on other accounts. 
g 2 
