276 On the Manufacture of Optical Glass. [1829. 



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 appli- 

 cation 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 diffi- 

 culty 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 day- 

 light 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 1J 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 triborate-of-lead glass is almost as colourless 

 as good flint glass, but might perhaps be found objectionable 

 on other accounts. 



99. As there is a certain quantity of light intercepted by 

 glass which is altogether dependent upon and in proportion 

 to its colour, it is evident that this property of the heavy 

 glasses must be considered in relation to their use in tele- 

 scopes ; but there appears no reason for supposing they will 

 ultimately prove inapplicable on this account. The colour of 

 the glass already obtained is far less in depth than that of the 

 crown glass constantly used in the construction of telescopes, 

 which yet intercepts by its colour no important quantity of 



