18 THE ROYAL SOCIETY OF CANADA 



scopic purposes and most of it would be suitable for use in construct- 

 ing achromatic combinations. 



There does not seem to be any direct connection between the trans- 

 missive properties for ultra-violet light and its physical characteristics, 

 such as flecks, clouds or starts. 



It would seem that we need no longer be dependent on foreign 

 sources of supply with such a source as the South African one available, 



April 15, 1919. 



