MODERN SUN CULT STURMER 199 



human skin connected with the production of vitnmin D are those 

 of wave lentj^ths from 2,900 A to 3,130 A. This region of the ultra- 

 violet rano^e of wave lenti^ths is known as the Dorno region, and 

 embodies the rays used therapeutically. These are the rays which 

 are desired chiefly filso in the radiation from the ultra-violet lamp. 

 Rays of wave lengths from 2.S00 A to 3,300 A are known to produce 

 pigment, but particularly is this effect said to be due to the wave 

 lengths of the Dorno region. It appears that tanning is nature's 

 protection against too much ultra-violet radiation of the wave 

 lengths which cause the chemical changes in the skin, for when the 

 (an has become deep enough, the Dorno rays no longer penetrate, 

 but are absorbed by the pigment and changed to heat rays. 



WINDOWS 



Window panes of quartz glass, SiO., give, of course, the best results 

 in the transmission of the ultra-violet of the Dorno region; but only 

 quite recently has the manufacture of quartz window panes been 

 j)laced on an industrial basis, and unfortunately the price is still 

 too high to admit of their general employment in the windows of 

 solaria. The other types of ultra-violet transmitting glass, though 

 not as satisfactory, are much cheaper. A still cheaper substitute is 

 celophane, which is essentially a transparent product of cellulose. 

 This substance transmits the radiation quite well, hut it lacks dura- 

 bility, and requires a specially constructed frame. 



DOSAGE OF SUNSHINE 



How much sunshine do we need? Sunshine, like heat, food, or 

 medicine, or any other agency which affects us, must be taken in 

 proper apportionment. There is such a thing as too much food. 

 Certainly, such a thing as too much calomel. That too much heat 

 hurts us, we learn in early childhood. Sooner or later we learn also 

 that too much sunshine is harmful. And an extensive and deep 

 sunburn is not only painful, but like any other burn, is dangerous, 

 resulting in cell destruction, with the generation of toxins, which 

 cause systemic poisoning. Thus even sunshine must be taken in 

 proper dosage. Fortunately, however, sunshine does not ordinarily 

 harm us if we accustom ourselves to it gradually, and thus avoid 

 sunburn. This remark, however, applies only to persons who are 

 well, for in certain diseases even strong sunshine may be detrimental. 



PROTECTION AGAINST ULTRA-VIOLET 



If we are interested in protection against ultra-violet rather than 

 in its transmission, we need but remember that substances which 

 allow only the longer waves of the visible spectrum to pass — amber 



