388 ANNUAL. REPOET SMITHSONIAN INSTITUTION, 19 3 2 



Glasses. — Crystalline quartz and freshly distilled water that has 

 not been in contact with glass are highly transparent to ultra-violet 

 rays, in 1 cm thickness transmitting about 80 per cent at 200 milli- 



s .a 



X o 



2 s. 



l-l 



I i s-2— T «: 



, L S. 2, 



-i-s-: 



I li'j 



microns. Quartz glass seems more opaque than crystalline quartz to 

 wave lengths less than 240 millimicrons. The ultra-violet trans- 

 parency of fluorite varies considerably from sample to sample, some 



