SUN'S RAYS 



83 



Freer, Gibbs, and Bacon have shown that, though the tropical 

 light of Manila contains few, if any, more ultra-violet rays than the 

 Temperate Zone, still the chemxical rays have more effect there 

 than in a temperate climate, and these effects vary on different 

 days. Freer distinguishes ' actinic ' and ' non-actinic ' days, on 

 both of which the sky may be equally clear. The cause of the 

 difference between an actinic and non-actinic day is not under- 

 stood. Gibbs does not believe that the normal intensities of the 

 light in the tropics is different from that of any other region, the 

 influence of local meteorological conditions being excluded, the 

 most important of these being the humidity. Aron suspects that 

 the deleterious influence of tropical sunlight is due to the long 

 heat waves rather than to the short chemical waves, which we will 

 now consider. 



The chemical rays appear to have, first, a stimulative and 

 beneficial influence, and, secondly, a harmful influence. 



Stimulative Influence.- — ^Finsen, by experiments upon tadpoles, 

 earthworms, beetles, flies, etc., came to the conclusion that the 

 action of the chemical or blue-violet rays was very considerable as 

 compared with light (yellow) or heat (red) rays, and that, though 

 their action was probably very complex, still, it could be best 

 considered as an excitation of the nervous system. 



This excitation was so powerful as to produce reflex actions in 

 tadpoles and movements in other animals, while in man he con- 

 siders them to be the cause of the feeling of bien-etre experienced on 

 a bright, sunny day, which he compares with the depression felt 

 on a dark, cloudy day. 



Bactericidal Properties.— Downes and Blunt in 1877 showed that 

 the chemical rays could kill bacteria, while D' Arson val and Charrain 

 showed that they could kill the Bacillus pyocyaneus. In 1903 

 Bernard and Morgan demonstrated that it was the middle third of 

 the ultra-violet portion of the spectrum which caused these bac- 

 tericidal effects. 



Harmful Influences- — iVcuxE Skin Irritation. — Charcot in 1859 

 first expressed the opinion that it was the chemical and not the 

 heat rays which produced sunburn, and showed that the dermatitis 

 caused by strong electric light was identical with that caused by the 

 sun. 



In 1889 Widmark proved this definitely by using an electric arc 

 of 1,200 candle-power, the light of which was first passed through 

 water to absorb the heat rays, and then allowed to fall upon the 

 shaven skin of a white rabbit, when the characteristic inflammation 

 resulted. He then interposed a plate of ordinary glass to exclude 

 the ultra-violet rays, when the skin remained unaffected. 



In 1 90 1 Finsen placed on the flexor surface of his forearm a plate 

 of rock-crystal and pieces of different-coloured glass, and also wrote 

 his initials in Indian ink. He then exposed the arm to the rays 

 from an 8o-ampere arc for twenty minutes, ten minutes at a distance 

 of 50 metres and ten minutes at 75 metres. The result was that 



