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 flocculi 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 peculiaritles in the relative forms of the hydrogen and calcium flocculi, corresponding to different velocities and distribution of the respective gases.
The Rumford spectroheliograph has also been used to secure photographs with some of the stronger dark lines of iron and other substances, which show the vapors of these metals on the sun. But even with the grating, the dispersion is insufficient to give thoroughly trustworthy 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 different levels, and the method of detecting local differences of temperature 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 monochromatic 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 180 feet long, and the importance of providing tor simultaneous photography in several lines at different parts of the spectrum, would require that