MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
47 
a little anxiety for the capability of resistance in the heavy glasses may readily 
be allowed, since they contain so much less of the substance (silica) which 
confers the power of resistance, and so much more of that (oxide of lead) 
which is considered as the vulnerable part, than does either of the former kinds. 
105. The superficial changes of glass which interfere with its optical uses 
are of two kinds. The one is shown by a tarnish upon the surface, which 
when strong is iridescent. It is quickly produced by the intentional presence 
of sulphuretted hydrogen, which acting upon the oxide of lead present, reduces 
it, and forms a sulphuret of lead. It takes place only with flint glass, and is 
in every case produced either by sulphuretted hydrogen or other sulphuretted 
vapours. In plate glass the change is of another kind, and is shown by the 
appearance of minute vegetations or crystallizations, which spread, obstructing 
the light wherever they occur. Mr. Dollond, who has shown me cases of 
both kinds of injury in flint and plate glass, is inclined to believe that the 
latter has, during his long experience, proved most injurious. 
106. From the commencement of the experiments it was expected that these 
heavy optical glasses would tarnish more than flint glass ; but as specimens of 
borate of lead and other dense compounds of that metal had been retained in 
an ordinary atmosphere, without any particular precautions, for long periods of 
time, yet without tarnishing, there was encouragement to continue the investi- 
gations: and though when specimens were put into atmospheres purposely 
contaminated with sulphuretted hydrogen, they tarnished quickly, and much 
more than any flint glass, yet it did not follow that they should of necessity 
tarnish in the telescope ; especially as, being from the construction of the achro- 
matic object-glass inclosed by the tube and the crown or plate glass lens, they 
would be considerably protected, and at the same time would admit of the in- 
tentional application of extraneous chemical protectors. 
107- The kind of protection which occurs to the mind is the application of 
such substances to the interior of the tube as, having a strong attraction for 
sulphuretted vapours, should continually retain the atmosphere within free 
from their presence. Carbonate of lead, precipitated borate of lead or finely- 
ground litharge, mixed with the pigment which is usually applied to blacken 
the inside of the telescope that all extraneous light may be absorbed, will pro- 
bably effect this purpose completely. 
