2 FREER. 



conducted upon the influence of this or that type of clothing, on the 

 modifications of covering upon troops in the field and other similar 

 lines of investigation always suffer from the factor of the condition 

 of the subjects themselves. The impossibility of introducing any exact 

 comparisons into these experiments renders the conclusions drawn 

 therefrom arbitrary. 



However, in almost all cases, differences in the environment brought 

 about by contrasts between tropical and temperate climates are referred 

 in the first instance to the sunlight, in the second to humidity, if this 

 factor is taken into consideration at all, and in the third to the equable 

 temperature throughout the year : that is, to the absence of a pronounced 

 winter. 



It has seemed to me that the first of these factors, the sunlight, is 

 capable of some exact comparative measurements, although, from the 

 nature of the case, the experimental difficulties in the way are very 

 great. To draw any final conclusion, work with reliable and calibrated 

 instruments must be undertaken in various parts of the globe, through 

 a long series of days, and experiments must be planned so as to eliminate, 

 so far as possible, the personal errors which may creep in. We must 

 take into consideration the average 1 number of hours of sunshine per 

 day, the degree of haziness or cloudiness, the variation with the seasons, 

 the presence or absence of pronounced winds and the proximity to the 

 seashore of the stations as well as their altitude. Needless to say that, 

 while many individual observations exist, no comprehensive plan of work, 

 based upon data taking cognizance of all of these factors, has ever been 

 carried into effect. In order, if possible, to make a beginning, I have 

 brought together the results of the investigations which we have been 

 able, so far, to conduct in this institution, together with such as may 

 be pertinent to the question obtained in other places. The sum total 

 leaves us with only a few facts gained and a few conclusions to be 

 draw-n, but it shows to a certain extent what direction it is best to 

 follow in cooperative work and it also reveals to us the complexity of 

 the field. 



THE SUNLIGHT. 



We all know that the sunlight can be dissolved by proper instruments, 

 such as a diffraction grating or a prism, into an infinite number of 

 waves or pulsations of different length, which produce a continuous 

 visible spectrum extending from deep red to violet; this spectrum being 

 crossed by numerous sharp lines, representing the absorption phenomena 

 of the elements existing in a gaseous state in the sun. On either side an 

 area not visible exists, the infra-red and the ultra-violet. The latter por- 

 tions of the spectrum can be studied by means of photographs. Measure- 

 ments are made in millionths of a millimeter, starting from some well- 

 known line which may be produced between two poles of a given metal by 



