SAMUEL PIERPONT LANGLEY. 519 



evidence of their structure. I learn from Mr. Abbot, of the astro- 

 physical observatory, that '"' Prof. G. E. Hale, who has enjoyed the 

 choicest opportunities for examining the sun, botli with the 40-inch 

 reflector of the Yerkes observatory^ and with the horizontal telescope 

 on Mount Wilson, and also during various expeditions to high moun- 

 tain peaks, says that in the best views of sun spots he has ever had 

 the better they Avere seen the more nearly have they appeared as 

 shown in Langley's drawings." In spite of this great power of 

 direct personal observation, he was quick to appreciate and to employ 

 the aids which photography lends to this research, though it should 

 be said that the standard illustration of a sun spot which appears 

 in most of the text-books and w'orks on astronomy of the present 

 time is one drawn by Mr. Langley witli his own hand at Allegheny 

 in December, 1873. The following statement of his continued work 

 in this field during his Allegheny period was prepared recently for 

 publication in a general encyclopedia, and, having had the advantage 

 of his own revision, it is taken as an authoritative statement of his 

 researches : 



About 1875 he began to devote much atteutlou to the measurement of the 

 heat spectra of the sun and other sources of radiation. Convinced after long 

 experience witli the thermojiilo of the futility of attempting to discriminate 

 the effects of narrow i)ortions of the spectrum by means of any heat-measuring 

 apparatus then employed, he sought to devise something more satisfactory, and 

 in 1879 and 1880 was successful in the invention of the bolometer. This 

 instrument has found high favor for a wide range of experimental work, but 

 in his hands it has been used from 1880 to the present time to open up a 

 great new field of investigation in connection with the invisible long wave- 

 length rays proceeding from all heated bodies and to change many of the older 

 ideas concerning them. 



The more important of his many researches published during this period were 

 upon the energy spectrum of the sun, the transmission of the earth's atmos- 

 phere and the solar constant, the behavior of prisms toward long wave-length 

 radiators, the energy spectra of heated terrestrial bodies, and the energy 

 spectrum of the moon, the moon's heat hitherto having been recognized with 

 difliculty even in gross by the thermopile, but now, by the bolometer, being 

 analyzed in minute detail in a lunar heat spectrum. More recently a com- 

 parison of the proportions of luminous and nouluminous heat in the spectra of 

 the sun and artificial light sources with the corresponding proportions of the 

 light and heat in the radiations emitted by the glowworm gave important 

 economical results. 



In 1881, previous observations at Allegheny having led him to believe that 

 there was a great and then unappreciated selective absorption both in the 

 sun's and in the earth's atmosphere, which rendered in the latter case Pouillet's 

 methods inapplicable, and which when recognized tended to give a far larger 

 value to the solar constant, he, with the aid of the Government, organized an 

 expedition to the top of Mount Whitney, the loftiest mountain in southwestern 

 California, whose abrupt [)recipices permitted observations to be made from 

 two neighboring stations, yet with a distance of more than 2 miles of altitude 

 between them. These observations were published by the United States Gov- 



