490 BIOLOGICAL EFFECTS OF RADIATION 



between the various wave-lengths of roentgen and gamma rays, and that 

 it may be possible to bring out this differential effect by the fractionated 

 method of dosage. One difficulty lies in the method of measuring the 

 dosage of the various wave-lengths over the whole range of the electro- 

 magnetic spectrum so that equivalent amounts of energy may be used. 

 Individual variation in the response of the animal or human skin is also 

 important, often as much as 20 per cent (88). 



The effects produced by the ultra-violet, visible, and infra-red radi- 

 ations depend upon the character of the substances making up the skin, 

 i.e., some of the radiation is reflected (363) and some is absorbed (44) by 

 the stratum corneum and pigments (142, 202, 343), the capillary bed 

 (363), and tissue proteins (76, 391). 



Bachem, in 1927 (9, 10), in a very extensive study of the transmissi- 

 bility of the skin, has found that the stratum corneum transmits best in 

 the middle of the ultra-violet spectrum with a secondary maximum near 

 2500 A, for the malphigian layer has an absorption maximum at the 

 border between the visible and the ultra-violet, so that the corneum 

 absorbs both the shortest and the longest infra-red radiations and trans- 

 mits the long ultra-violet rays and the near infra-red rays. The sub- 

 cutaneous tissue receives only long wave-lengths (down to depths of 2 to 

 3 cm.) centering about 6000 A, (9, 10, 53) while red light penetrates quite 

 well through fingers, ears, cheeks, sinuses (frontal). In the infra-red 

 transmission rises around 10,000 A and falls around 14,000 A, The 

 development of pigment or an erythema or ischemia retards the transmis- 

 sion of the ultra-violet radiation (87, 152, 202, 340), but in general the 

 reaction is proportional to dosage and large doses of all but the visible 

 cause destructive effects similar to that produced by roentgen rays 

 (173, 174). The skin temperature may rise (even with visible radiation) 

 owing to the reflex dilation of the skin capillaries (53, 142, 175, 328, 330). 

 The reaction is magnified, where the source is sunlight or a carbon arc, 

 by the deeper effects of the more penetrating short infra-red and red 

 radiation, which cause dilation of the larger vessels beneath the skin. 

 This brings about an erythema (when mild doses are used) beyond the 

 area irradiated (142) which is of longer duration that that due to the 

 longer infra-red and ultra-violet radiation absorbed in the skin proper. 



Dosage and the absorption spectrum of tissues are thus both important 

 in these effects. Human skin has erythema maxima at 2500, 2800, and 

 3000 A (133, 139), and at 3030 A (357, 358), and reflects very little of the 

 wave-lengths between 3000 and 4000 A. Pigment development occurs 

 strongest at 2480 and 2540 A, less at 2970 and 3030 A (330), but the 

 matter of equal dosage for such experiments has not been verified, for 

 either erythema or pigment production. 



In general, the physical character of the absorbing surface or tissue 

 (skin) determines the effectiveness of individual wave-lengths of the 



