406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
accident and still others have grown out of the ancient experience and wisdom 
of the race. 
Physical medicine, in particular, is the outgrowth of experience. 
Apparently heat and cold, air and water, and the beneficial influence 
of the sun’s radiation have been used to fight diseases since the earliest 
times of humanity. The powerful effect of radiation therapy is known 
to everyone who has used this treatment in too large dosages on the 
beach ; painful sunburn is the vehement reaction of the body. On the 
other hand, lack of the sun’s radiation can cause severe diseases, since 
the formation of important compounds in the skin is due to photo- 
synthetic processes and requires the presence of radiation of the ap- 
propriate wave length. 
The wave lengths of light which are of greatest importance in medi- 
cine are located in the ultraviolet portion of the spectrum. In general, 
three different ranges of ultraviolet radiation may be distinguished. 
The range of from 315 to 400 millimicrons has the power to penetrate 
the skin to an appreciable extent and is used in the treatment of some 
diseases, for instance, lupus. A second range of ultraviolet radiation, 
between 280 and 315 millimicrons, produces erythema of the human 
skin and is applied in the treatment of rachitis (rickets). The range 
of wave lengths shorter than 280 millimicrons has a particular germi- 
cidal effect. The primary effect of ultraviolet rays on cells or on pro- 
teins is still a difficult problem, especially since it is expected that the 
energy of a single photon of the radiation must have some relation to 
the binding energies within the structure of proteins. 
Such specific effects are not to be expected in the infrared part of 
the spectrum. The effect of infrared treatment consists more of super- 
ficial heat, which may increase the flow of blood through the capillaries, 
relax muscle spasms, or produce locally increased metabolism. Fre- 
quently one finds the opinion expressed that infrared rays are able to 
penetrate into the depths of the body, but this is not so. Particularly 
those lamps used in infrared therapy which operate at low-color tem- 
perature (room heaters, for example) produce a radiation of which not 
more than a fraction of 1 percent enters the skin. The wave lengths 
shorter than 0.7 micron are absorbed by oxyhemoglobin, those longer 
than 1.4 microns by the water content of the tissue. Of course, this 
does not mean that infrared treatments are ineffective but rather that 
beneficial effects occur only in certain regions of the body. 
If heat is to be applied into the deeper regions of the body, the 
techniques of diathermy and short-wave diathermy are more appro- 
priate. Short-wave dielectric and induction heating have been used 
in medicine for more than a decade and have been employed by in- 
dustry on a large scale for only a few years. Industrial research 
