A NEW METHOD OF RESEARCH. 21 



a moment's consideration will show that tie- 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 in- 

 tensity 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 experi- 

 ence 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 physi- 

 ological 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 Eumford 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 



