200 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 



glass, red glass, orange, and yellow glass — will prove opaque to 

 ultra-violet, while a medium which cuts out the red end of the 

 spectrum and transmits the violet and blue will prove to be rather 

 transparent also to the adjacent invisible waves just beyond the 

 violet. Thus blue cobalt glass transmits some ultra-violet, while 

 red glass and amber glass filter it out, together with the short-wave 

 radiation of the visible spectrum, namely, the violet, blue, and green. 



It will be seen, in the light of this fact, that the practice, formerly 

 in vogue, of keeping the light-sensitive chemicals in blue glass 

 bottles was a misguided procedure, destined to prove ineffectual, 

 while our present-day practice of employing amber glass is founded 

 on sound principles and is much more effective. 



Another means employed to shut out ultra-violet is to interpose 

 a fluorescent substance as a protective, for such a substance will 

 transform this radiation into visible light waves. The cold creams 

 and ointments specially devised to protect against sunburn are com- 

 pounded on this principle and contain fluorescent material. Even 

 the ointment bases are fluorescent. 



That sunlight deleteriously affects house paint is well known, and 

 we have all observed how much longer paint lasts on the north 

 side than it does on the southward exposure. The inclusion of a 

 fluorescent material is, therefore, of benefit in house paint. Now 

 zinc white is such a substance, and its employment in the paint 

 materially lowers the penetration of the ultra-violet rays which 

 are so destructive to the linseed-oil binder, causing the oxidized oil 

 to disintegrate to powder, thus bringing about the result which 

 the painter refers to as " chalking." Most face paint, also, is 

 somewhat fluorescent, and is to some degree a protection against 

 sunburn. 



CLOTHING AND ULTRA-VIOLET 



A few remarks about clothing may be relevant in this discussion. 

 A person exposed to the ultra-violet in connection with electric 

 welding may receive a burn through his clothing. This is due to 

 the intensity of the radiation. But most clothing effectively inter- 

 cepts the light rays and also the ultra-violet radiation of sunlight. 

 Animal hair is quite opaque to solar ultra-violet. Only very white 

 hair transmits it to a slight extent. This means that all woolen 

 clothing is an effective ultra-violet filter, and that the modern male 

 promenading on the boardwalk receives practically no such radia- 

 tion through his clothing. Silk also is fairly opaque, linen a trifle 

 less so, whole cotton is a bit more transparent. The most trans- 

 parent textile is a loosely-woven rayon or artificial silk, which 

 explains why the sex known during the Victorian era as the 

 weaker sex is on the way to becoming the robust sex. The girls 



