6o CARNEGIE INSTITUTION 



In addition to the bolometer, referred to elsewhere, the two prin- 

 cipal instruments employed for this work in conjunction with the 

 telescope are the spectroscope and the spectroheliograph. The 

 former permits the investigation of the nature of the chemical ele- 

 ments in the sun, their physical condition, and their motions in the 

 direction of the earth, while photographs taken with the latter show 

 the distribution of the various elements in the solar atmosphere and 

 on the sun's disk. The two instruments supplement each other 

 most effectively, and can be used to great advantage in connection 

 with the bolometer and other physical apparatus. 



The sun closely resembles the earth in chemical composition, but 

 differs from it in every other particular. Intense heat, indefinitely 

 greater than that of a Bessemer converter, maintains its substance 

 in a state of vapor. The visible surface of the sun marks the limit 

 where the metallic vapors of the interior, coming into contact with 

 the cold of space, condense into luminous clouds. This surface 

 presents a granular appearance, since the bright clouds form the 

 upper extremities of columns of vapor ascending from the interior, 

 separated by spaces filled with cooler and less luminous vapors. 

 Above the visible clouds columns of uncondensed vapors continue 

 to ascend. Hydrogen, helium, and calcium rise to heights of several 

 thousand miles, and at certain points project from the nearly con- 

 tinuous sea of flame (the chromosphere) in the form of great gase- 

 ous prominences, ranging in altitude from 15,000 to 300,000 miles. 

 At times of greatest solar activity violent eruptions frequently occur, 

 producing prominences which sometimes rise to a height of nearly 

 300,000 miles in less than half an hour. 



Formerly the flames of the chromosphere and prominences were 

 visible only at the sun's circumference, when at times of total 

 eclipse the dark body of the moon inter\^ened to cut off the over- 

 powering illumination of the earth's atmosphere. In 1868 it was 

 found that the)' could be observed in full sunlight with the spectro- 

 scope, and in 1892 they were first successfully photographed with 

 the spectroheliograph. This instrument also permits the flames to 

 be photographed in projection against the sun's disk, thus render- 

 ing possible the investigation of a great variety of new and remark- 

 able phenomena. The invisible vapors of calcium, and recently, 

 through the application of the Rumford spectroheliograph of the 

 Yerkes Observator)^ those of hydrogen, iron, magnesium, or any 

 other substance present in the chromosphere, can be photographed 

 at will, and their forms, distribution, and motions investigated. 



