108 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 2 



little more improved, the desert regions of the world could supply 

 cheap solar power in quantities beyond the extremest possibilities of 

 future world demands. A lesser, but still interesting aspect of solar 

 radiation, lies in its use for domestic purposes. Food ma}^ be cooked, 

 and water may be heated by simple and inexpensive contrivances for 

 utilizing solar radiation. 



It may be that none of the services rendered to us by the sun 

 is more important than its esthetic use. The colors of all flowers 

 represent the fragments of the complete solar spectrum remaining 

 after the pigments of the plant world have absorbed certain rays. 

 Unabsorbed remainders ai'e reflected and produce gorgeous mixtures 

 of colors. Without the green of the grass and trees, the blue of the 

 skj^, the brilliant hues of liowers, and the more somber, but yet 

 pleasing shades of the soil, and thousands of familiar objects on all 

 sides, we should be sad indeed. 



Though thus obviously so important, it is little more than a 

 century since measurements began to be made of the intensity of the 

 solar radiation. Pioneers in this investigation were C. S. M. Pouillet, 

 Sir John Herschel, and J. D. Forbes. They all devised instruments 

 adapted to absorb the yolar radiation as completely as possible and 

 thus to convert it into heat. By appi-opriate devices for measuring 

 the heat thus produced, they obtained measurements of the intensit}' 

 of solar radiation. So intimate is the association between radiation 

 and heat that many people have contused the two. Yet they are 

 distinct and different. Radiation coinprises a mixture of impulses 

 which traverse a vacuum at the enormous speed of 186,000 miles 

 per second. These impulses are separable by prisms or gratings into 

 innumerable regular periodicities. These periodicities produce dif- 

 ferent sensations of color in our eyes. Many of them, indeed, are 

 invisible to us. Since radiation can traverse the vacuum and never- 

 theless seems to comprise transverse waves of exceeding shortness, 

 philosophic minds have not been content to contemplate so great 

 a paradox as waves traveling in nothingness. They have devised 

 the idea of the luminiferous ether as a medium filling all space. At 

 present, our ideas are in a state of flux regarding this abstruse subject, 

 but at least we can think of radiation as something which can exist 

 where there is no matter in the ordinary sense, for it comes to us 

 across an immense void from the sun and the stars. Heat, on the 

 contrary, is well recognized to be the motion of the molecules of 

 material substances. The energy of radiation is competent to stir 

 up this mode of motion called heat when, for instance, radiation is ab- 

 sorbed by a black object. If the object is " absolutely black," ab- 

 sorption is complete and all the energy of the ray is transformed 

 into heat. 



