12 SMITHSONIAN MISCELLANEOUS COLLECTIONS. 



purpose ; it finally enabled him to measure differences in temperature 

 less than one-hundred-thousandth of a degree centigrade, and 

 it proved to be the precursor of instruments giving even a more 

 astonishing degree of accuracy. It was speedily applied by astron- 

 omers and physicists in wide ranges of experimental work, and in 

 Langley's hands was used from the time of its invention down to the 

 last days of his life, especially in opening up a great new field of in- 

 vestigation in connection with the invisible long wave-length rays 

 proceeding from heated bodies, and has been a main means of 

 developing a new science of these. 



The more important of his many researches published in the 

 period which now began were upon the Energy Spectrum of the 

 Sun, the Transmission of Light through the Earth's Atmosphere, 

 the Solar Constant, the Behavior of Prisms toward long Wave- 

 Lengths, the Energy Spectra of Heated Terrestrial Bodies, and the 

 Energy Spectrum of the ]Moon. Hitherto the moon's heat had been 

 recognized with difficulty, even in gross, by the thermopile ; but now, 

 by the bolometer, it was analyzed in minute detail, in a lunar heat 

 spectrum. All this suggested later a comparison of the proportion 

 of luminous and non-luminous heat in the spectrum of the sun and 

 artificial light sources and a multitude of supplementary researches. 



During the summer of 1878 he took charge of a party sent out by 

 the United States to study from Pike's Peak the total eclipse of that 

 year, and was able to follow the sun's corona to a distance from the 

 main body hitherto unsuspected ; during the winter of the same 

 year he visited Europe and followed up these observations by others 

 upon Alount Etna. 



In 1 88 1, through the generosity of citizens of Pittsburg and with 

 the cooperation of the United States Signal Service, he conducted 

 an expedition to Mount Whitney, the loftiest mountain in southern 

 California, and among the pregnant results of his observations was 

 a volume which established the present view regarding the selective 

 absorption of the sun's light by our atmosphere. These researches, 

 being presented in a brief minor paper and an extensive memoir, 

 attracted wide attention both at home and abroad. 



In 1885 he had followed the actual solar spectrum to wave- 

 lengths ten times as great as those of the visible spectrum. 



Subsidiary to these researches were those upon the optical charac- 

 teristics and possibilities of rock-salt, thus developing most usefully 

 ]\Ielloni's experiments with this substance in his epoch-making re- 

 searches nearly a hundred years before. 



The reputation thus gained by Langley came rapidly and increased 

 steadily. The thoroughness, ingenuity, and beauty of his methods 



