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, relation 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 temperatures 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 methods 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 comparisons 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 impartial manner. The Bureau of Science at Manila is very fortunately situated in regard to this work. Its equipment is ample and the composite 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 institution for cooperation. The opportunity being at hand, it seemed advisable to begin cooperative work on the subject of tropical sunlight.