102 ARON. 



Because of the more nearly vertical incidence of the sun's rays in the 

 Tropics, a greater proportion reaches the earth and with a greater in- 

 tensity on a given area than in the northern and southern latitudes. 

 Of course, in granting this, such phenomena as cloud formation are 

 excluded. 



The tropical sunlight, in so far as the violet and ultra-violet end of 

 the spectiTim is concerned, has been studied extensively in the past 

 few years in the Bureau of Science in Manila by Freer,^ G-ibbs,^ and 

 Bacon,* and the effects produced by this portion of the sunlight have 

 been and are being compared with those obtainable by observers using 

 the same means of measurement in other latitudes. These investigations 

 up to the present have shown that the spectrum of the sun's rays do3S 

 not extend much, if any, farther into the ultra-violet in Manila than 

 in northern climates. Observations carried on daily during the year 

 on the decomposition of a solution of oxalic acid under the influence 

 of uranyl acetate as a catalyzer ^ have shown as great variations 

 between individual days, even of the same apparent brightness, and 

 some decomposition even on cloudy and rainy days; however, with a 

 general tendency toward maximum decomposition when the sun is nearest 

 the zenith and of minimum under opposite conditions. The com- 

 parative measurements in other countries are not as yet available to any 

 extent, with one exception. Bacon showed that the decomposition in 

 Manila in July was from five to twenty times greater than in Chicago 

 in June. 



The work with the ultra-violet spectrum was of interest not only because 

 it is necessary thoroughly to consider these rays in a study of tropical 

 sunlight, but also because of the number of authors,'' especially in modern 

 times, who are inclined to the belief that the action of the tropical sun on 

 the human organism is to be attributed to the influence of the rays of 

 shorter wave length. As a result of this belief a special underwear, 

 which by its color should be impermeable to the rdtra-violet rays, has 

 recently been recommended for use in the Tropics. 



An extensive investigation of the relation of the color of underwear 

 to the health of men in this climate was made by Phalen and Nichols '' 



^This Journal, Sec. B (1910), 5, 1. 



UUd., Sec. A (1909), 4, 133; (1910), 5, 9 and 419. 



*Ibid. (1910), 5, 267. 



"The solution of oxalic acid uranyl acetate is only acted upon by the ultra- 

 violet end of the spectrum. The results of this work will be published later 

 from the Bureau of Science. 



"Woodruff, C. E., The Effects of Tropical Light on White Men. New York 

 and London, (1905); Duncan, Journ. Roy. Army Med. Corps (1908), 11, 71; 

 Simpson, Ibid., 441-449; Gihon, Twentieth Century Practice of Medicine, New 

 York (1895), 3, 253-285. 



'This Journal, Sec. B (1911), 6, 526. 



