LIGHT THERAPY MAYER 387 



SOME PHYSICAIi PROPERTIES OF LIGHT 



Workers vary in their divisions of the ultra-violet into the near or 

 long ultra-violet and the far or short ultra-violet. Clinicians for 

 convenience often term those ultra-violets longer than 290 millimi- 

 crons (the lower limit of sunlight) as the near or long ultra-violets; 

 while those shorter than 290 millimicrons are then called the far or 

 short ultra-violets. This is the designation used in this article. The 

 boundary line for physicists is often taken at 200 millimicrons be- 

 cause above this wave length it is possible to use ordinary photo- 

 graphic plates, quartz lenses and an apparatus open to the atmos- 

 phere. Below this limit, other means must be used. Some workers 

 have even based the differentiation on transmission of the rays or the 

 lack of transmission through window glass, the long or near ultra- 

 violets penetrating window glass, the far or short being absorbed by 

 it. 



The spectrum of sunlight shows the visible region limited on one 

 end by infra-red and on the other by ultra-violet rays (fig. 1). 

 The visible spectrum extends from about 760 to about 390 millimi- 

 crons. The lower limit of the visible region may vary with different 

 individuals according to the sensitivity of the retina. The ultra- 

 violet rays of sunlight extend from about 390 to 290 millimicrons, 

 and those of clinically used artificial sources from about 390 to below 

 200 millimicrons. The ultra-violet rays of a source such as the plain 

 carbon arc of 20 amperes, which has been so frequently used clin- 

 ically, extend to about 220 millimicrons (the intensity can vary 

 with amperage and with impregnations in the carbons), while those 

 of the quartz mercury-vapor arc terminate at 185 millimicrons. 



All wave lengths of radiation appear to possess some ability to 

 produce heat and to influence chemical reactions. As a rule, how- 

 ever, heat production is associated chiefly with the infra-red and 

 the red rays. Light is associated with the rays from red to violet, 

 while the most active rays chemically are the ultra-violet. The 

 green region (500 millimicrons) is strongest in sunlight and best 

 reflected by chlorophyll. 



The infra-red rays longer than 1.4 microns are penetrating except 

 for water, while the ultra-violet rays shorter than 0.3 micron are 

 strongly absorbed." The regions of the spectrum vary greatly in 

 their degree of absorption in different substances.® AVater and water 

 vapor, for instance, absorb infra-red rays to a great extent except 

 that region close to the visible from about 760 millimicrons to 1.4 

 microns. The ultra-violet rays lose the greater part of their intensity 

 in penetrating the atmosphere to reach the lowlands. 



"Aschkinass, E., Zeitschr. fiir Psychologie und Physiologic der Sinnesorgane, vols. 11 

 and 12, p. 43, 1896. Vogt, A., Schweiz. med. Wclinschr., vol. 55, p. 425, May 14, 1925. 

 •Kimball, H. II., Monthly Weather Rev., vol. 47, p. 769, 1919. 



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