July 5, 1901.] 



SCIENCE. 



9 



portant epocli in this class of investigations 

 from the fact that photography was now 

 for the first time generallj'^ applied. The 

 photographs possessed the great advantage 

 of freedom from personal bias and of 

 forming a permanent record which could be 

 studied at leisure. Although nothing was 

 previously known of the character of the 

 rays with whfch the impression must be 

 taken, as it happened the results were 

 eminently satisfactory. The comparison 

 of plates taken hundreds of miles apart 

 showed identically the same forms, thus dis- 

 posing of the notion that they were due 

 to personal or atmospheric causes, while 

 those taken at the same place, in close suc- 

 cession, showed the moon to pass over them, 

 gradually covering or uncovering them as 

 the case might be. 



The eclipse of 1868 was distinguished by 

 another great advance in the practical ap- 

 plication of the spectroscope. JSTow for the 

 first time the true character of the so-called 

 prominences was demonstrated, viz., that 

 of glowing gases or vapors shooting up to 

 heights of fifty or a hundred thousand 

 miles above the sun's surface, and com- 

 posed in great part of hydrogen. A con- 

 spicuous line was also seen near the D line 

 of sodium. As this corresponded to no 

 chemical element then known, it was 

 called the helium line. In 1895 helium 

 was discovered in a gas obtained from the 

 mineral cleveite, an interesting case of a 

 chemical element first discovered in the sun. 



In connection with this eclipse it was 

 found that these prominences could be ob- 

 served at any time when the sun was 

 visible by a proper use of the spectroscope. 

 This important discovery was hit upon 

 independently by M. Janssen and Mr. 

 Norman Lockyer. Both discoverers com- 

 municated their methods to the French 

 Academy, the letters reaching the Secre- 

 tary within a few minutes of each other. In 

 commemoration of this event a medal was 



prepared bearing the effigies of both Janssen 

 and Lockyer. 



The principle employed in obtaining the 

 images of the prominences is as follows : 

 The light of these objects is largely mono- 

 chromatic. If such a ray is passed through 

 a prism it is bent out of its course, losing a 

 little of its brightness by the absorption of 

 the glass but not otherwise. The light due 

 to the glare of the atmosphere, however, 

 which is the cause of our inability to see 

 these features whenever the sun is visible, 

 being composed of all the colors of the spec- 

 trum, is dispersed and rendered so faint as 

 not to interfere with the image of the 

 prominence. The higher the dispersion, 

 the darker is the background against which 

 this image is seen. It was at first thought 

 necessary to employ a narrow slit, thus 

 gradually building up the prominence by 

 taking narrow slices in succession. It was 

 soon found that the slit could be opened 

 wide enough to show the entire image at 

 once. This discovery made possible the 

 careful and deliberate study of this feature 

 of the sun, with the result that more is 

 probably known of it than could ever have 

 been ascertained, had it been necessary as 

 at first to employ only the few moments 

 during total eclipse. 



The next step in advance would seem to 

 be in the direction of accomplishing for the 

 corona what had been done for the promi- 

 nences and thus make possible the study of 

 this feature of the sun's environment under 

 the same leisurely and deliberate condi- 

 tions. This problem has received a great 

 amount of attention during the past twenty 

 years. Various methods of attack have 

 been suggested and tried, but so far without 

 success. 



In 1882 Professor Huggins believed that 

 he had solved the problem. By the use of 

 plates sensitive only to the most prominent 

 rays of the corona, he obtained photographs 

 which had every appearance of being genu- 



