OP ARTS AND SCIENCES, 511 



concave mirror, and were concentrated upon the bolometer. The 

 deviation of the rays was measured by a divided circle, and the wave- 

 lengths calculated. No collimator was used, and there was no ab- 

 sorption except by the atmospheres of the sun and the earth. The 

 absorption by the earth's atmosphere was calculated by comparing 

 noon observations with those taken near to sunrise and sunset. The 

 absorption of violet light was great, and of the most luminous rays 

 greater than for red light or the infra-red waves. The inference was 

 drawn that outside of our atmosphere the sun would be bluish. 



Armed with his new and powerful instrument of research, Professor 

 Langley went on a scientific expedition to Mt. Whitney, in Southern 

 California, in 1881, with the purpose of measuring the solar constant 

 of radiation and the absorption produced by the earth's atmosphere ; 

 occupying, successively, three stations at 800, 4000, and 4800 meters 

 of elevation, where the rarity and dryness of the air promised a 

 greater degree of accuracy in his work. Observations were made 

 with the actinometer of Violle and the pyrheliometer of Pouillet 

 synchronously with those of the bolometer. With a Hilger glass 

 prism of the finest quality and workmanship and a Rutherford grating, 

 for the prismatic and normal spectra, the bolometer revealed to his 

 intellectual eye a region of invisible solar activity outside of the red 

 greater than that which the eye had ever seen. 



On his return to the Allegheny Observatory he found that he could 

 continue his work at this lower level with advantage, since his study 

 of atmospheric absorption enabled him to eliminate its influence upon 

 his measurements. He corrected the errors introduced into the 

 prismatic spectrum by the absorption of a glass prism, by repeating 

 the observations with prisms of rock-salt, fluor-spar, and quartz. In 

 the spring of 1882 he repeated his work upon the normal spectrum 

 with one of Professor Rowland's admirable gratings ruled upon a 

 concave surface of metal. It contained 18050 lines, 142 to the mil- 

 limeter, and exposed a ruled surface of 129 centimeters. Being of 

 short focus, it gave a specially hot spectrum. 



The general conclusions which had been reached were these : that 

 light, heat, and chemical action coincided in the curves which expressed 

 the distribution of these various forms of energy ; that absorption was 

 inversely as the wave-length ; that the greater energy in the luminous 

 region was there in spite of its greater absorption ; that the maximum 

 energy was near the yellow ; and that the new district which had been 

 added to the normal spectrum was greater than the visible and invisi- 

 ble spectrum which had been known before. Moreover, the general 



