LIGHT THERAPY MAYER 389 



specimens, from 2 to 3 mm in thickness, transmitting as low as 80 per 

 cent at 230 millimicrons. Ordinary window glass absorbs practically 

 all ultra-violet radiations shorter than 320 millimicrons. Certain 

 kinds of speciall}'^ prepared glass transmit varying percentages of 

 ultra-violet rays; clean vitaglass 2 mm in thickness, for example, 

 transmits 75 per cent of the ultra-violet rays at 320, 25 per cent at 

 290, and about 5 per cent at 270 millimicrons. At normal incidence 

 about 9 per cent of the energy between 700 and 400 millimicrons is 

 lost on account of the reflection of light at the surfaces of the glass, 

 increasing to 20 per cent for an angle of incidence of 60°.^ Most 

 window glasses which are made especially for transmitting short 

 wave length ultra-violet radiation season for the first few weeks of 

 usage, losing a certain amount of transparency to ultra-violet. 

 When this point is reached, the transmissibility is fixed and per- 

 manent, and they still transmit an effective quantity and quality of 

 ultra-violet light for antirachitic effect. This has been shown for 

 December in New York City with three hours' daily exposure in the 

 middle of the day.® Corex-D glass (Corning, N. Y., Glass Works) 

 transmits the solar ultra-violet rays more fully than other glasses 

 except quartz, but its cost still makes its use prohibitive for any but 

 research purposes. 



Various thin and porous cloths treated with thin films of paraffin, 

 and wire mesh screens filled with celluloidinous material transmit 

 the solar ultra-violet and visible radiations very well, especially 

 when these materials are freshly prepared. The ultra-violet rays 

 between 320 and 290 millimicrons are probably the best pigment- 

 producing rays; because of their absorption by window glass it 

 becomes difficult to tan behind such glass. Sand, snow, ice, and 

 water increase the intensity of the ultra-violet by reflection ; however, 

 the dense humidity over bodies of water may intercept the reflected 

 ultra-violet rays. 



SOME BIOLOGIC EFFECTS OF LIGHT 



Red and infra-red rays if of sufficient intensity produce an im- 

 mediate hyperemia, which, however, soon disappears after the 

 cessation of the irradiation. Infra-red rays longer than 1.4 microns, 

 it has been shown, are more likely, through their superficial absorp- 

 tion, to produce cutaneous blisters in animals; those shorter than 

 1.4 microns penetrate the skin. The visible rays provoke vision, 

 heat, and metabolic changes ; ^ the red rays are able to elevate sub- 

 cutaneous temperature as much as 3° C. without causing bodily 



■^ Coblentz, W. W., and Stair, R., Bur. Standards Journ. Res., vol. 3, p. G29, 1929. 

 «Eddy, W. H., Report from Teachers College, Columbia University, New York City, Oct. 

 9, 1929. Eddy, W. H., Science, vol. 68, p. 13, 1928. 



» Sonne, Carl, Arch. Physical Therapy, vol. 10, p. 239, June, 1929. 



