junction with the telescope. The principle of the method is simple and easily understood. The white light of the sky, when passed through a spectroscope, is drawn out into a long rainbow band, and thereby enormously reduced in intensity. The light of the prominences, on the contrary, is concentrated in the radiations characteristic of hydrogen and helium gas, and the great dispersing power of the spectroscope merely separates more and more widely the colored images which correspond to these radiations, without greatly reducing their intensity. With the spectroscope they therefore become visible, since their images are brighter than the highly dispersed background of skylight on which they lie.
Armed with this method, observers in various parts of the world have systematically observed the forms of the solar flames on every clear day, giving us a continuous record of these phenomena now extending back for more than thirty years. From a study of this record many conclusions regarding the nature of the flames and their bearing on the question of the solar constitution, have already been reached. But the process of observation is not only slow and painstaking; it is also subject to the errors and uncertainties that attend the hand delineation of every object, seen through a fluctuating atmosphere, under unfavorable conditions. It was principally in the hope of simplifying this process, and of rendering it more rapid and more accurate, that the spectroheliograph was devised by the writer in 1889.
The principle of this instrument is very simple. Its object is to build up on a photographic plate a picture of the solar flames, by recording side by side images of the bright spectral lines which characterize the luminous gases. To accomplish this an image of the sun is formed by the telescope on the slit of a spectroscope. The light of the sun, after transmission through the spectroscope, is spread out into a long band of color, crossed by lines representing the various elements. At points where the slit of the spectroscope extends out beyond the sun's edge across a gaseous prominence, the bright lines of hydrogen and helium may be seen extending from the base of the prominence to its highest point. If a series of images of such a line, corresponding to different positions of the slit on the prominence, were recorded side by side on a photographic plate, it is obvious that they would give a representation of the form of the prominence itself. To produce such an effect it is only necessary to cause the solar image to move at a uniform rate across the first slit of the spectroscope and then, with the aid of a second slit, which takes the place of the eyepiece of the spectroscope, to isolate one of the lines, permitting the light from this line, and from no other portion of the spectrum, to pass through the second slit to a photographic plate. If the plate is moved at the speed with which the solar image passes across the