LIGHT THERAPY MAYER 401 



lessened heat makes this tolerable. Hence, by the law of inverse 

 squares, close exposures increase the quantity of light utilized. As a 

 result, lesser amperage arcs, such as 29 amperes, with specially im- 

 pregnated carbons, may prove as effective on close irradiation of 

 single patients as high amperage arcs necessarily operated at greater 

 distance and generally emploj^ed for groups. 



Other artificial sources of light are frequently employed, especially 

 in the European clinics. These include arcs between electrodes of 

 tungsten, iron, magnetite, or cadmium; also oxyacetylene flames. A 

 recent development is a combination of a mercury arc between highly 

 incandescent electrodes of tungsten, all inclosed in a special glass 

 (Corex) that absorbs the ultra-violet shorter than 280 millimicrons, 

 not present in sunlight.*^ The total intensity at about 3 feet distance 

 is about one-twelfth average solar radiation. About 1.5 per cent of 

 the total radiation emitted consists of radiation of wave-lengths less 

 than 313 millimicrons. 



A so-called cold quartz light, also recently developed, is a glow dis- 

 charge through mercury vapor, emitting mostly short ultra-violet 

 rays shorter than those present in sunlight (90 per cent or more at 

 254: millimicrons). The exact place of these sources of light in 

 therapeutics must still be fixed. 



Indications for the use of various sources of light therapy are 

 still inexact, as are also the contraindications ; but in many localities 

 in cold and cloudy seasons of the year, when sunlight is too uncertain 

 and solar exposures are too interrupted, artificial sources of light 

 have proved valuable aids and substitutes. 



PIGMENT 



Mercury-vapor arcs produce a yellowish-brown pigmentation. 

 However, at times I have observed mercury-vapor lights produce a 

 pigment barely distinguishable from that produced by sunlight, but 

 it does not last as long. Pigment produced by intense sources of heat, 

 such as oxyacetylene blasts or osram lights, is said to simulate sun- 

 light tan closely, and to be just as permanent (Kisch). This is 

 doubtful. The pigment from sunlight, the carbon arc and tungsten 

 arc is blackish brown or red-brown. 



That increased pigment production necessarily means an increased 

 tendency to healing is not generally accepted. Similarly, however, 

 the maintained production of an erythema is not a necessary accom- 

 paniment of improvement. Pigment can protect against an overdose 

 of ultra-violet energy; it absorbs light, changing it to heat, and may 

 allow for better heat radiation; it is doubtful whether it sensitizes 



" Luckiesh, M., Artificial Sunlight, New York, D. Van Nostrand Co., 1930. 



