328 ANNUAL BEPORT SMITHSONIAN INSTITUTION, 1925 



hand, if visible rays alone are concentrated on the skin to the just 

 bearable degree, the temperature just beneath the skin will be raised 

 even to 47° C. This result, confirmed by Argyll Campbell and L. 

 Hill, is due to the greater absorption of dark heat by the surface 

 layer of the skin and a deeper penetration of the visible rays. Sonne 

 ascribes heliotherapy to the local heating effect of the visible rays 

 and has tried to find evidence that such local heating of the blood 

 increases specific antibodies of the body — e. g., the diphtheria anti- 

 toxic content of the serum. However, P. Hartley has reinvestigated 

 this matter with great refinement and accuracy of method, both in 

 regard to the diphtheria antitoxin content of the serum and the 

 agglutinin content of the serum against B. typhosus, and finds that 

 light baths have not the least effect on such a specific immunity. 

 The baths have power, on the other hand, to increase the general 

 resistance of the body to infection, as was shown by L. Colebrook, A. 

 Eidinow, and L. Hill, who found that a light bath intense enough to 

 produce erythema put up the haemo-bactericidal power of the blood 

 as tested in vitro. Blood which before the bath killed say 80 per 

 cent of staphylococci mixed with it, two hours after a light bath 

 killed 100 per cent. Such an effect followed no less when a lasting 

 erythema was produced by exposure to heat or a mustard poultice. 



In the case of the light bath the relative activity of visible and 

 ultra-violet rays is proved in the following way : If an arc light with 

 "• Avhite flame " carbon poles (direct current and about 2,500 kilo- 

 watts) is focussed sharply through a quartz lens on to the arm, an 

 unendurable burning sensation results almost at once. If the arm is 

 immersed in a quartz vessel full of cold water and the experiment 

 is repeated, no burning sensation results, but if the exposure is con- 

 tinued for 5 minutes, erythema develops some hours later at the ex- 

 posed spot, and this may advance even to a blister, to be followed by 

 a long, lasting, brown pigmentation. Repeating the last experiment 

 with a quartz screen filled with 3 per cent quinine solution inter- 

 posed between the arc and the arm, all ultra-violet rays shorter than 

 330 ix}jL are thus cut out, as can be shown by the quartz spectrograph. 

 In this case no erjiihema results even after over an hour's exposure. 

 The visible rays then, apart from their heating effects, have no effect 

 on the skin. The ultra-violet rays, acting on the cooled skin, have, 

 on the other hand, a profound effect. 



Using a quartz spectroscope and a blackened thermopile for meas- 

 uring the energy of various parts of the ultra-violet spectrum, it was 

 found by Hausser and Vahle that the maximal power for producing 

 erythema of the skin was with the wave-lengths 300-290 /iju,, just the 

 region wliicli comes through with the high sun on clear days. Little 

 effect was given by rays 313 /x/t and 250 /n/t. A screen of uric acid 



