154 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



Smithsonian Institution, and was its director until his death in 1906. 

 His own principal investigations, and those of the Astrophysical 

 Observatory begun under his direction and stUl continued, have lam 

 m the field of measuring the quantity and quality of the sun's radia- 

 tion, the effect of the earth's atmosphere thereon, and the dependence 

 of terrestrial temperatures and plant life on solar radiation. This 

 is a utilitarian branch of astronomy, whose applications to terrestrial 

 concerns may be expected to increase in future years and result in 

 the promotion of the arts of meteorology and agi-iculture. But the 

 interest of such studies for the promotion of pure knowledge is also 

 very high. Let us imagine that the Greek philosophers, the Ara- 

 bians, and the astronomers of GalUeo's time, had all possessed the 

 means to measure accurately the quantity and quality of solar radia- 

 tion. How interesting it would be now to compare then- measure- 

 ments with our own, and determine thereby what, if any, appreciable 

 changes have occurred m 2,500 years in that energy which supports 

 heat and life upon the earth! The astronomer of the futaire will 

 have, we hope, trustworthy measurements of our own time to compare 

 with his own. Referring to another brancli of the measurements 

 which I am to bring before you, our knowledge of the approximate 

 temperatures prevaUmg in the sun, and our conclusions as to the 

 sun's nature rest on such work as is being done at the Smithsonian 

 Astrophysical Observatory. 



By the term solar radiation, I propose to your minds not only the 

 solar rays which affect our eyes as light, but the extensions of the 

 spectrum beyond the violet and beyond the red, where the eye is 

 not sensitive. All these rays, whether visible or not, may be absorbed 

 by blackened surfaces and will thus produce their just and propor- 

 tional effects as heat. For the measurement of solar radiation, 

 Langley, about 1880, invented the delicate electrical thermometer 

 shown in plate 2, which he called the bolometer; figure 8 of plate 2 

 shows its most important part. This is a pair of tapes of platinum, 

 each about 1 centimeter long, 0.01 centimeter broad, and 0.002 

 centimeter tliick. These tapes are blackened with camphor smoke 

 or by a deposit of platinum black. One is exposed in the path of 

 the rays to be measured, and the other is hidden. Hence one tape 

 is warmed with respect to the other. Thereby a mmute electrical 

 current is caused to flow through the delicate galvanometer con- 

 nected with the Wlieatstone's bridge, of which the tapes form two 

 arms. In tins way a change of temperature, which may be as small 

 as one-millionth degree Centigrade, may be detected in ordmary 

 practice. By special devices the sensitiveness may be mcreased 

 beyond this one-hundred fold. But though so sensitive the bolo- 

 meter is far behuid the eye in its capacity to detect faint yellow 

 light. It is used in preference to the eye because it can detect and 



