This method, when tried by the writer at the Harvard Observatory in 1890, proved unsuccessful. The lack of success was partly due to the fact that a line of hydrogen was employed. This line, though fairly suitable for the photography of prominences with the perfected spectroheliograph of the present day, was too faint for successful use
amidst the difficulties which surrounded the first experiments. Accordingly, when the work was resumed a year later at the Kenwood Observatory in Chicago, an attempt was first made, through a photographic investigation of the violet and ultra-violet regions of the prominence spectrum, to discover other lines better fitted for future experiments. In the extreme violet region, in the midst of two broad dark bands which form the most striking feature of the solar spectrum, two bright lines (H and K) were found which were attributed to the vapor of calcium. They had previously been seen visually in the prominences, but on account of the insensitiveness of the eye for light of this color, their true importance had hardly been realized. A careful study soon showed them to be present in every prominence observed, at eleva-
