THE BIOLOGICAL ACTION OF LIGHT ^ 



By Prof. Leonard Hilt., F. R. S. 

 From the National Instilute for Medical Research 



While the heat spectrum, including infra-red, visible and ultra- 

 violet rays, extends from a wave-length of some 60,000 /xfi ^ to one of 

 100 ixfj,, beyond the outer dark heat rays are the Hertzian waves used 

 in radio with wave lengths extending to a thousand meters or more. 

 The inner dark heat rays merge into the visible, Avhich are Avave 

 lengths from 700 fifi (red) to 400 i^ix (violet). Beyond the visible 

 lie the invisible ultra-violet raj's with wave length from 400 fx/x to 

 100 fifx, and beyond these come the soft X-rays and then the hard 

 X-rays and the y rays of radium with wave lengths so short as 

 0.01 fifi. 



The body of a man is surrounded with a horny layer of skin 

 beneath which lies the living cells of the epidermis in thin layers and 

 myriad in number. Beneath them circulates the blood through close 

 Avoven networks of capillaries, in streams some 0.01 mm. thick. The 

 epidermis reflects and scatters rays which fall upon it, but some part 

 of the visible rays penetrate and are absorbed by the blood beneath, 

 Avarming it. The dark heat rays, on the other hand, absorbed mostly 

 by the epidermis, warm it more than the blood in the dermis. Set as 

 windoAvs in an almost impenetrable skin, the eyes have been evolved 

 Avith extreme sensitivity to a narrow portion of the spectrum^— 

 namely, the visible rays. To Hertzian waves we are insensitive ; their 

 energy has to be converted into sound and heard. Likewise we can 

 not feel ultra-violet, X or y rays; a latent period of two or three 

 weeks folloAvs exposure to X or y rays before an erythema of the 

 skin and irritation result. A latent period also follows exposure to 

 ultra-A'iolet rays, but one limited to hours. While the hard X-rays 

 and y-rays and secondary rays started by these penetrate in part to 

 deep tissues, the active ultra-violet rays are Avholly absorbed by the 

 epidermis and exert their effect there. 



Sonne found that if dark heat rays are brought to bear on the 

 skin up to the just endurable sensation of burning, the temperature 

 just beneath the skin Avill be raised to about 43° C. On the other 



1 Substance of two lecturos given at the Royal Institution on Mar. 12 and 10 ; reprinted 

 by permission from Nature, No. 2896, vol. 115, May 2, 1925. 

 = ^^=0.000001 mm. 



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