198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



Now ultra-violet, speaking generally, is much less penetrating 

 than the longer waves which we can see, and this feeble penetrability 

 diminishes more and more as the wave decreases in length. The 

 opacity of most known substances to the ultra-violet radiation has 

 tended to make its study difficult, and it was not until the technique 

 of making quartz glass had been perfected that much headway could 

 be made. But in fused quartz we have a medium transparent to 

 waves as short as 1,850 A. For the study of still shorter waves cer- 

 tain minerals, namely fiuorite and calcite, are employed. But the 

 waves shorter than 1,850 A have at present no practical application, 

 so the quartz glass serves very acceptably. 



THE WAVE LENGTHS WHICH PASS THROUGH THE ATMOSPHERE 



Solar radiation is rich in ultra-violet. Just how rich, and what 

 its range of wave lengths may be, we do not know, perhaps shall 

 never know, for our atmosphere, even when it is clear, cuts out all the 

 wave lengths shorter than those of about 2,900 A. What is known 

 concerning the shorter waves has been learned through the study of 

 radiation artificially supplied by arc lights and lights from sparks 

 through metallic vapors. 



ULTRA-VIOLET THROUGH GLASS 



Common glass transmits only the longest waves of ultra-violet 

 radiation, those which are longer than about 3,400 A, and hence are 

 in the neighborhood of the visible violet light waves. This explains 

 why a sunporch furnishes impoverished sunlight. But in recent 

 years special kinds of glass have been made, designed to transmit 

 the solar ultra-violet more generously. The claim is made that this 

 glass transmits about 65 per cent of the invisible active rays, a 

 statement which requires explanation. The facts are these: Not 65 

 per cent of all the ultra-violet waves of sunshine pass through in 

 the same proportion in which they occur out of doors, but rather 65 

 per cent of the radiation, with a much greater absorption of the 

 shorter waves — even a total absorption of some of them — and a 

 lesser interception of the longer ones. In other words, these so- 

 called ultra-violet transmitting media do cut out an appreciable 

 amount of the wave lengths shorter than about 3,000 A. 



THE DORNO REGION 



The significance of the interception of wave lengths shorter than 

 about 3,000 A by the special glasses, and of all wave lengths below 

 3,300 or 3,400 A by common glass is better understood when it is 

 known that the rays which produce the beneficial effects on the 



