280 On the Manufacture of Optical Glass. [1829. 



more of that (oxide of lead) which is considered as the vul- 

 nerable 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 hy a tarnish 

 upon the surface, which when strong is iridescent. It is quickly 

 produced hy 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 vege- 

 tations 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. 



10G. 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 investigations ; 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 would of necessity 

 tarnish in the telescope ; especially as, being (from the con- 

 struction of the achromatic object-glass) enclosed by the tube 

 and the crown or plate glass lens, they would be considerably 

 protected, and at the same time would admit of the intentional 

 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 pre- 

 sence. 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 probably effect this purpose completely. 



108. A very curious and important influence of alkali in facili- 



