a moment's consideration will show that no serious difficulty need arise from this cause. For the darkness of the lines is only relative; if they could be seen apart from the bright background of continuous spectrum on which they Lie, these lines would shine with great brilliancy. It is thus evident that if all light except that which comes from one of these dark lines can be excluded from the photographic plate by means of the second slit of the spectroheliograph, it should he possible to obtain a photograph showing the distribution of the vapors corresponding with the line in question.
At this point attention should he called to the extreme sensitiveness of the spectroheliograph in recording minute variations in the intensity of a line—variations so slight that no trace of them can be seen in a spectrum photograph showing only the line itself. A well-known physiological action is here concerned, for it is common experience that the eye can not detect minute differences of intensity in various parts of an extremely narrow line, whereas these would become much more conspicuous if the line were widened out into a band of considerable width. The action of the spectroheliograph is to record side by side upon the photographic plate a great number of images of a line which, taken together, build up the form of the region from which the light proceeds. In this way the full benefit of the physiological principle is derived, and very minute differences of intensity between various parts of the solar disk are clearly registered upon the photographic plate.
It is obviously essential in photographing with the dark lines to exclude completely the light from the continuous spectrum on either side of the line employed. The admission of even a small quantity of this light might completely nullify the slight differences of intensity recorded by the aid of the comparatively faint light of the dark line. As the second slit can not be narrow r ed beyond a certain point, it is evident that for successful photography with the dark lines their width must be increased by dispersion in the spectroheliograph to such a degree as to make them wider than the second slit.
The first successful photographs obtained with dark lines were made with the Rumford spectroheliograph in May, 1903. The lines of hydrogen were chosen for this purpose, on account of their considerable breadth, and because of the prominent part played by this gas in the chromosphere and prominences. In order to secure sufficient width of the lines, the mirror of the spectroheliograph was replaced by a large plane grating having 20,000 lines to the inch. After leaving the grating the diffracted light enters the prisms, where it is still further dispersed before the image of the spectrum is formed upon the second slit. The effect of the prisms is not only to give additional dispersion, but also to reduce the intensity of the diffuse light from the grating