346 Trowbridge and Hutchins — Carbon in the Sun. 



One of Rowland's concave gratings, of 21 feet 6 inches in 

 curvature and 14,000 lines to the inch, was employed. In 

 order to avoid any possible displacement of the photographic 

 camera during the operation of photographing the carbon 

 spectrum immediately below the solar spectrum, a drop shut- 

 ter was arranged directly in front of the sensitive plate, the 

 movement of which was independent of any movement of the 

 camera. Preliminary experiments showed us the importance 

 in this work of employing a spectroscope of great dispersion 

 and of fine definition, giving also a normal spectrum. The use 

 of a prism spectroscope would undoubtedly have masked the 

 phenomena we have observed. For our purpose, therefore, 

 Rowland's apparatus was peculiarly advantageous. 



Our experiments lead us to conclude that there is positive 

 evidence in the solar spectrum of the existence of carbon in the 

 sun. Before giving an account of our experiments in detail, a 

 few observations may not be considered out of place. 



One who studies the solar spectrum by itself, and who has 

 had no experience in the formation and observation of metallic 

 spectra, is apt to regard the dark lines in the solar spectrum as 

 fixed in character and condition. A line which is seen by one 

 observer, and not by another, is generally regarded as a terres- 

 trial line formed by absorption in the earth's atmosphere. 

 Certain lines are well known to be due to the terrestrial absorp- 

 tion, as can be easily proved by their appearance when the sun 

 is observed at sunset, when the rays of light have to penetrate 

 a greater thickness of the earth's atmosphere than at midday. 

 The shifting layers of vapor in the sun's atmosphere also may, 

 in certain cases, obliterate or strengthen certain lines of metal. 

 To understand this it is only necessary to extend the reasoning 

 of the conservation of energy to the subject. It is a common 

 lecture experiment to reverse the metallic lines by passing the 

 rays of light produced by the vapor of the element through a 

 layer of vapor colder than that of the source of the rays. The 

 energy of the rays is thus absorbed in heating the colder layer. 

 When the temperature of the vapor is increased, and becomes 

 equal to that of the source, no reversal takes place. Thus, on 

 the sun's surface the conditions for a reversal may be wanting 

 at certain times, and faint lines may become bright. Their 

 brightness may not be sufficient to affect the general illumina- 

 tion of the solar spectrum of which they form a part. Condi- 

 tions may arise, moreover, in which the temperature of the 

 reversing vapor may be called critical, — at such a temperature 

 that the faint reversal is sufficient to extinguish the bright line 

 of a metal without producing a well-defined dark line. At cer- 

 tain epochs, also, the temperature of the vapor of any element 

 in the sun may be higher than at any other time ; and certain 



