202 Al^^NUAL REPORT SMITHSONIAN INSTITUTION, 193 



VARYING INTENSITY OF SUNLIGHT 



It may, further, help us to guard against sunburn if we remember 

 the fundamental fact that the lower strata of our atmosphere are 

 particularly absorptive toward ultra-violet, much more so than the 

 more rarified strata higher up, the curve of ultra-violet in sunshine 

 being virtually identical with the arc described by the sun in its 

 course from horizon to horizon, which explains why old Sol is so 

 much more powerful near the middle of the day, and much feebler 

 when in the morning and evening its rays travel a longer distance 

 through denser strata. It is indeed a common observation that at 

 sunrise and at sunset even the violet and blue of the visible spectrum 

 is largely cut out, giving us a preponderance of reddish and orange 

 light. And as a matter of fact, the ultra-violet is almost wholly in- 

 tercepted at the same time. 



WINTER SUNSHINE 



In the winter, the sun is not so high in the sky, and its slanting 

 rays are for this reason relatively poor in ultra-violet, just as are 

 those of the morning and evening sun. Furthermore, there may be 

 fog, clouds, and soot particles, all of which filter out the shorter rays. 

 Soot is the greatest offender in this respect, and does much to deplete 

 still further the enfeebled sunshine of the cold season of the year. 

 Our industrial centers naturally suffer most in the matter of vitiated 

 sunlight, for in order that we may keep warm, and that our wheels 

 of industry may be kept spinning, immense quantities of soot are 

 belched forth from our chimneys. 



DEFICIENCY OF ULTRA-VIOLET 



In other words, we are systematically sacrificing the short-wave 

 solar ultra-violet in order that we may have an abundance of the 

 longer waves which keep us warm. Only in very recent years have 

 we learned about the unhealthfulness of this practice, and have 

 come to; the conviction that something must be done about it. A 

 paucity of solar radiation means, so medical men tell us, a slackened 

 metabolism, an enfeebled condition, a lowered resistance to germ 

 diseases. It has in fact been suggested that the gradual extinction 

 of certain Indian tribes may be due largely to their altered dress, 

 and their changed modes of life, resulting in inadequate sun irradia- 

 tion, which is so necessary to them. In children, regardless of race, 

 too little sunlight may mean rickets, or tuberculosis; and it was in- 

 deed in the treatment of these diseases that solar radiation was first 

 employed in a systematic way. The sun cure is not, however, in 

 reality modern, for Antyllus, a physician of ancient Rome, made use 



