522 THE POPULAR SCIENCE MONTHLY 



zones, although this statement may sound paradoxical, and this fact in 

 turn may morphologically modify the peoples to a greater extent than 

 other influences. 



It can readily be understood that a place recognized to be within 

 the tropics, may, by reason of its proximity to the sea, its altitude, rela- 

 tion to mountain chains, and other natural surroundings, have a climate 

 so modified that the actual sunlight may have less influence than in 

 localities which may be situated upon the borders or even within the 

 temperate zones. Another factor influencing local conditions may be 

 the color of the soil and the resulting modification of the intensity of 

 the heat rays coming from it, because the radiation from the soil is of 

 importance. 



In other words, pronounced differences may be found between the 

 climates of two places in the tropics which may geographically be close 

 together, as may readily be seen by comparing the meteorological data 

 from Alexandria, Cairo and Aswan, in Egypt. Prevailing winds may 

 so modify the climate of a region that during the nights the tempera- 

 tures may closely correspond to those found in places more favorably 

 placed. Indeed, in comparing two regions we may find the anomaly 

 that the tropical situation, for long periods, may have a more temperate 

 climate than is found at the place nominally lying outside of the zone 

 in question. 



The general trend of the discussion in the literature has been that 

 the rays of the sun lying in the region of the spectrum comprising the 

 violet and ultraviolet are of the greatest importance in determining the 

 influence of insolation upon human beings, and these rays have a 

 special, though undeserved, designation as actinic rays. Studies of the 

 influence of these rays have not been lacking; it has clearly been shown 

 that they are destructive to great classes of microorganisms, and meth- 

 ods have also been devised to measure the relative proportion of these 

 rays as compared with other parts of the spectrum. However, no 

 thoroughly systematic work in this line has been carried out, and com- 

 parisons in the tropics are decidedly lacking. 



Therefore it seems evident that the entire question of the influence 

 of insolation upon the inhabitants of the tropics exists as a legitimate 

 field for experimental study and that comparative data from different 

 regions may be obtained so as to solve many of the questions in an im- 

 partial manner. The Bureau of Science at Manila is very fortunately 

 situated in regard to this work. Its equipment is ample and the com- 

 posite nature of the staff makes it possible to carry it on in a number of 

 lines simultaneously. In addition, the newly organized University of 

 the Philippines offers opportunity to call upon the faculty of that insti- 

 tution for cooperation. The opportunity being at hand, it seemed ad- 

 visable to begin cooperative work on the subject of tropical sunlight. 



