396 Transactions of the Society. 
as to the precise nature of the glasses used seems to be forth- 
coming. 
The subject of the permanency of some kinds of Jena glass is of 
great interest ; but while it is highly desirable that all the elements 
in an objective should be of lasting and untarnishable character, it is 
nevertheless a fact that much can be done by judicious protection of 
any doubtful ones. The boro-silicate flint of Schott & Co. tarnishes 
very rapidly in ordinary air, on account of the presence of minute 
traces of sulphur compounds. This flint, when enclosed between two 
other glasses in Dennis Taylor's telescope objective, has been found to 
show no perceptible alteration after five years’ use. ; 
