BIOLOGICAL ACTION OF LIGHT — HILL 331 



Campbell and L. Hill). Pith helmets prevent local heating of 

 the head and sunstroke in the tropics. In the West Indies, these 

 are not needed owing to the screening of the sun by water vapor. 

 There is rarely any danger of sunstroke in Great Britain, The sun 

 is powerfvd enough only on few summer days, and bare heads offer 

 no risks except on these rare occasions. Heatstroke from overheat- 

 ing of the whole body due to exhaustion of sweating in warm 

 stagnant air is much more common — overclothed as we are for with- 

 standing hot weather. 



Downes and Blunt (1877) proved that the bactericidal action of 

 light was due to ultra-violet rays, and much has been made of this. 

 It has been claimed recently by Wiesner that the infra-red rays, 

 apart from their heating effect, have a bactericidal action, but this 

 is not so (A. Eidinow and L. Hill). The only rays which kill, 

 apart from any lethal heating effect, are the ultra-violet rays. The 

 bactericidal power of these rays is not nearly so important as has 

 been thought, for the rays can only kill the surface bacteria. They 

 can not penetrate into filth any more than through the epidermis. 

 In their curative effect on lupus these rays act, not by directly 

 killing the bacilli, but by increasing the immunizing powers of the 

 tissues. This is so, exen when the rays are focussed as in the local 

 Finsen light treatment. Nodules so treated, when injected into 

 guinea pigs, produce tuberculosis. 



Mr. J. E. Barnard, by photographs taken with ultra-violet rays 

 and a quartz-lensed microscope, has shown structures hitherto un- 

 revealed in living yeasts, bacteria, and blood cells. This is due 

 to the selective absorption by the outer membrane, the nucleus, 

 and certain other granules in the cells. Infusoria vary in their 

 susceptibility to the lethal effect of ultra-violet rays, and this prob- 

 ably bears a relation to the age, nutritional state, and absorptive 

 particles within them. The lethal power on cells increases wath 

 shortening of the ultra-violet rays; for example, using the cadmium 

 spark, a 20-minute exposure sufficed to kill infusoria placed in the 

 275 fxix band, and a 3-minute exposure in the 232 fx/x band. The 

 penetration of the shorter rays, however, is far less, and these there- 

 fore have no action on the skin. Thus, as stated above, while 

 bands 275 and 257 fi/x of the cadmium spark produce erythema of 

 the skin, the intense 232 /x/x, band has no effect. 



In the case of the very short rays, Mr. J. E. Barnard finds one 

 anthrax bacillus screens another lying beneath it. With rays of 

 weak intensity, processes of repair may keep pace with injury, and 

 no effect be produced in living cells. A screen which allows ultra- 

 violet rays to pass so as to give an excellent spectrum, as photo- 

 graphically recorded by means of the quartz spectrograph, may be 



