THE TROPICAL SUNLIGHT. 17 



of the light diminishing with the density of the clouds. The Philippine 

 Weather Bureau has stated to me that, so far as they are aware, twenty- 

 four hours never pass on the Island of Luzon entirely free from cloud, 

 and if we calculate the average number of hours of sunshine in Manila 

 as compared with those of many other places in northern climates, such 

 as Denver or Santa Fe, situated in the Middle West of the United States, 

 we find the figures very must less for this district, namely, for Manila, 

 51 per cent of the theoretical sunshine as against 69 per cent for Denver 

 and 76 per cent for Santa Fe. Even Chicago and New York present 

 figures higher than Manila, namely, 57 per cent for Chicago and 56 per 

 cent for New York. While, during the clear days, the sunlight may be 

 more intense here, nevertheless many northern climates have, on the 

 average, more hours of insolation, entirely regardless of the lengthening 

 of the days during the summer months as we leave the equator. If we 

 take an entire year we may not have a greater amount of radiant energy 

 here than in many places of the Temperate Zone, and perhaps even less. 



To what, then, are the supposed untoward effects of the tropical 

 climate due? Is it perhaps that at certain times of the year we may have 

 a greater intensity of light, although the average may not be any more? 

 If this were true, would not the total result be even more apparent in 

 such regions as New Mexico, or in deserts, in which certainly the sum 

 total of insolation would be larger in quantity than in Manila? Un- 

 doubtedly, periods of great light intensity would, during certain times, 

 have their effect in the more rapid destruction of lower organisms, and 

 on plant life, on which only a brief period of excessive insolation would 

 be either destructive or highly detrimental, but is this true of a highly 

 organized being such as man, who, for a large part of the year, is pro- 

 tected, to a greater or less extent, by meteorologic conditions? The 

 results of intense insolation do not seem to be so very apparent in this 

 region ; certainly cases of sunstroke are rare. 



Much has been done in the wa}' of experiment with clothing and head 

 coverings of various colors, so as to avoid these very same effects, but in 

 introducing any such modification the experimental work of Baly and 

 his co-workers ~ in regard to the absorption spectra for ultra-violet of 

 the various dyes must be taken into account. It does not matter so 

 much what the color of the dye is, so long as it presents a considerable 

 absorption band in the proper region of the spectrum. 



It seems to me that in any discussion of the effects of the Tropics, not 

 only the sunlight, but also the general average humidity, the proportion 

 of cloudy and rainy days, and above all, the continued equable tem- 

 perature without sharp contrasts of heat and cold, must be taken into 



'■Journ. Chem. Hoc. London (1904), 85, 1029; (1905), 87, 1332; (1900). 89, 

 502, 966, 982; (1907), 92, 1572. 

 94620 2 



