OF ARTS AND SCIENCES. 11 



spectrum, a drop shutter was arranged directly in front of the sensi- 

 tive 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 phenomenon we 

 have observed. For our purposes, 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 charac- 

 ter and condition. A line which is seen by one observer, and not by 

 another, is generally regarded as a terrestrial line formed by absorp- 

 tion in the earth's atmosphere. Certain lines are well known to be 

 due to the terrestrial absorption, 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 a 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 experi- 

 ment to reverse the metallic lines by passing the rays of light pro- 

 duced 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. Conditions 

 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 certain epochs, also, the temperature of 

 the vapor of any element in the sun may be higher than at other 

 times ; and certain lines may thus appear which are wanting when 

 the temperature falls. One is forced to these conclusions in observing 



