STUDY OF THE EFFECT OF TROPICAL SUNLIGHT. HI 



spectrum, except possibly the ultra-violet, than it is in the low- 

 lands. This may be accounted for by the larger proportion of 

 cirrus clouds and haze and the dust-laden atmosphere of the 

 lower altitudes.'^ 



The absence of clear, blue skies over th(> tropical seas and 

 islands is not fully realized until experimentally demonstrated. 

 For about eighteen months I watched each day, in Manila, for 

 clear, blue skies for the purpose of taking photographs with the 

 rays of the spectrum lying between 690 n^i and 740 /x/y.,'' which 

 give the effect of silvery foliage again.st a black sky, and it was 

 on rare occasions only, due to the haze or cirrus clouds in good 

 weather, that successful photographs could be taken. In Baguio 

 the conditions required for this work occur quite frequently. 

 The large umbrella or the sun helmet again demonstrates its 

 usefulness as a protection to the eyes from the glare of the 

 lowlands. 



In a later article I hope to consider the question of the protec- 

 tion of colored glasses from the standpoint of their absorption 

 spectra. 



SUMMARY. 



In this paper the measurements of the skin temperatures in 

 the tropical sun, of a number of different races, the temperatures 

 under the hair and under various kinds of clothing, and the 

 subcutaneous temperatures of monkeys and rabbits together with 

 some observations of the physiological effects of sunlight are 

 described. 



In the shade, the skin temperatures of human beings remain 

 constantly below blood temperature. In the sun the temper- 

 atures of the lighter-colored skins sometimes rise more rapidly 

 than those of the darker colors, but, after the initial rise, the 

 darker colors maintain higher maxima than the lighter, pro- 

 vided the exposure of the lighter-colored skins is not too 

 long. Usually temperatures of the darker-colored skins rise 

 more rapidly. In the case of too long exposure an irrita- 

 tion of the sensory nerve-endings, nerve-endings in the vessel 

 walls, or of the vessel walls themselves, produces a flushing 

 of the skin due to a greater quantity of blood and a more rapid 

 flow. This eft'ect is absent in the darker skins the pigmentation 

 of which is undoubtedly a protection in this regard. 



"Blue color of the sky. Lord Rayleiprh, Phil. Mag. (1871), 41, 107, 

 274, 447 and (1899) 47, 375; Bauer and Moulin, Radium, 7. 372 through 

 Chem. Abstracts (1911), 5, 3642. 



"Wood, loc. cit. 



