September 16, 1898.] 



SCIENCE. 



349 



the sun's surface has forever disposed of the 

 alleged frequent transits of intra-mercurial 

 planets, so will the photographic plate 

 finally, when it has attained more per- 

 fection in dealing with the planets, show 

 that many of the strange features ascribed 

 to the surfaces of some of them do not exist. 

 As I have said, the early photographs of 

 eclipses seem to have been made with the 

 sole end in view of securing pictures of the 

 solar prominences. This was very impor- 

 tant at first, for by the photographs it was 

 proved that they were true solar phe- 

 nomena. Possibly also in the very first 

 photographs a picture of the corona was 

 considered a hopeless matter on account of 

 the lack of sensitiveness of the plates. 

 The eclipse of 1868 is memorable for having 

 shown Janssen and Lockyer that the visi- 

 bility of the prominences did not necessarily 

 depend upon a total eclipse of the sun. 

 They found that by the aid of the spectro- 

 scope the prominences could be seen and 

 studied at any time. This was an extremely 

 important step, and placed our knowledge 

 of the nature of the prominences upon a 

 firm and lasting footing. The fact that 

 these objects could be made visible with 

 the spectroscope soon suggested to Pro- 

 fessor Young the idea that they might also 

 be photographed at any time ; and in 1870 

 he made efforts to secure impressions of 

 them upon the photographic plate, and met 

 with partial success. To photograph suc- 

 cessfully these objects, however, required the 

 invention of a special instrument, for the 

 older methods must necessarily result in 

 failure. In observing the prominences 

 visually with the spectroscope it is neces- 

 sary to examine them through a slit which 

 is very narrow compared with the height of 

 the prominences. Only a small section of 

 the prominence can, therefore, be seen at 

 once, and to see it all the slit must be moved 

 over the prominence. If the slit is widened 

 more of the object is shown, but at the same 



time such a flood of light is admitted that it 

 is lost in the glare. If an instrument could 

 be devised whereby the slit could be moved 

 in front of a photographic plate, successively 

 exposing to the plate all parts of the promi- 

 nence, it will readily be seen that the entire 

 image could be photographed. To do this 

 there must be two slits moving in perfect 

 unison — one placed across the sun in front 

 of the grating or prism, the other in front 

 of the photographic plate and adjusted per- 

 fectly to the spectral line of the prominence 

 so as to exclude all light save that emitted 

 by the prominence itself, and thus, by the 

 gradual motion of these two slits, the entire 

 object is successively uncovered and an ex- 

 act photograph secured of it. 



The solar prominences consist mainly of 

 incandescent hydrogen and calcium. The 

 best results are secured by calcium alone. 

 It is curious to see photographs of the same 

 prominence made by using the hydrogen or 

 calcium lines independently; these pictures 

 often differ considerably, thereby showing 

 the peculiarity of distribution of the calcium 

 and hydrogen in the same prominence. 

 The two components are differentiated, 

 and it is thus shown just what part each 

 component takes in the composition of the 

 prominence. To make one of these pictures 

 takes several minutes of exposure, during 

 which time the slits slowly travel over the 

 region of the prominence. This extremely 

 ingenious device owes its existence to the 

 inventive genius of Professor Hale, who 

 devised and built the first instrument of 

 this kind, and secured the first actual spec- 

 troscopic photograph of the prominences. 

 These first pictures were made in 1891. 

 It is, therefore, now a matter of no great 

 labor to make not only photographs of 

 single prominences at any time, but through 

 a further ingenious extension of the possi- 

 bilities of the instrument it is made to move 

 across the entire sun's disc, thus secur- 

 ing every prominence at that time visible. 



