24 POPULAR SCIENCE MONTHLY. 



higher ones, and thus assist toward an understanding of the true cause 

 of the apparent darkness. A satisfactory comparison of the forms of 

 the hydrogen fiocculi with those of the calcium flocculi can only he 

 made after the question of level has been solved. Although there 

 is a general resemblance in form, as may be seen by reference to 

 Figs. 10 and 11, the differences are nevertheless striking enough to 

 suggest that various researches, interesting on physical and chemical 

 grounds, should be undertaken in the future. For instance, a series 

 of simultaneous photographs, in both hydrogen and calcium lines, 

 taken at brief intervals during the course of a violent eruption, might 

 show interesting joeculiaritles in the relative forms of the hydrogen 

 and calcium flocculi, corresponding to different velocities and dis- 

 tribution of the respective gases. 



The Eumford spectroheliograph has also been used to secure pho- 

 tographs with some of the stronger dark lines of iron and other sub- 

 stances, which show the vapors of these metals on the sun. But even 

 with the grating, the dispersion is insufficient to give thoroughly trust- 

 worthy results, except in a very few cases. It is evident that much 

 greater dispersion must be employed if the full capacity of the method 

 is to be brought out in future work. 



It is perhaps worth while to consider what are the logical steps to 

 be taken in the future development of the spectroheliograph. As at 

 present used, it is capable of solving a large number of problems if 

 employed systematically to register the changing forms of the calcium 

 and hydrogen flocculi. The method of photographing sections at dif- 

 ferent levels, and the method of detecting local differences of tempera- 

 ture or of physical or chemical state, should also permit important 

 knowledge to be gained. But to stop at the point reached, when there 

 is so much of promise in the further perfection of the method, would 

 commend itself to no one interested in the advancement of research. 

 The principal requirements of an instrumental nature are: 



1. Greatly increased dispersion in the spectroheliograph, through 

 the use of prisms or gratings in conjunction with collimator and 

 camera lenses of considerable focal length. 



2. A focal image of the sun at least twenty inches in diameter, of 

 which zones at least four inches wide can be photographed in mono- 

 chromatic light. 



These considerations would point to the use of spectroheliographs, 

 of from 12 to 40 feet focal length, provided with large gratings or 

 with three or four prisms. Such instruments would necessarily be 

 mounted in fixed positions on massive piers. A solar image 20 inches 

 in diameter would involve the use of a telescope about b80 feei 

 long, and the importance of providing tor simultaneous photography 

 in several lines at dill'ereni parts of the spectrum, would require that 



