THE STUDY OF TROPICAL SUNLIGHT. 21 



evident when these results are compared with those observed 

 for human beings that, all other things being equal, the Negro 

 will suffer more from the heat effects than the lighter-skinned 

 races. 



Chamberlain, =" in a series of observations in which he care- 

 fully compared the relative resistance to the Philippine climate 

 of blond and brunette types of soldiers, concludes that the 

 evidence is conflicting and that from a consideration of all the 

 facts the blonds are quite as well able as the brunettes to with- 

 stand the Philippine climate. The effect of the rays of greater 

 refrangibility in the violet and ultra-violet portions of the 

 spectrum are not the important factors, except in so far as 

 they cause sunburn and subsequent excessive pigmentation, 

 but protection from these rays is so easily accomplished and 

 has been accomplished so long as man has worn clothes, that 

 skin-color can not be an important factor in determining adapt- 

 ability to climate; that question is a morphologic one which 

 takes into consideration many more factors than skin-color 

 alone. A white cotton shirt and white trousers are sufficient 

 to protect against sunburn, and hence against the ultra-violet 

 rays. 



Phalen ^- compared 500 troops in the Philippines, dressed in 

 orange-red underclothing with 500 dressed in white. The ex- 

 periments show that the test underclothing added materially 

 to the burden of heat upon the system and that the white 

 underclothes of practically the same weight were superior in 

 this respect. In fact the lighter and whiter the clothing, the 

 better is it adapted to protection against the sunlight; indeed, 

 in the Tropics, were it possible, the ideal protection simply 

 would be an umbrella. The lowering of temperature in man 

 is brought about by evaporation of perspiration, and the better 

 the facilities offered for this purpose, the better off will the 

 individual be. 



Consequently, relative humidity plays a most important 

 part in the study of the influence of the sunlight. The higher 

 the relative humidity, other things being equal, the less readily 

 will evaporation take place and the less complete will be the 

 result in lowering the temperature. As the lowering is brought 

 about by the evaporation of sweat, it necessarily follows that 

 those races with the best developed sweat glands will have an 



" This Journal, Sec. B (1911), 6, 427. 

 '' This Journal, Sec. B (1910), 5, 525. 



