A NEW METHOD OF RESEARCH. 7 



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 prom- 

 inences, 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 ex- 

 tending 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 de- 

 lineation 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 re- 

 cording side by side images of the bright spectral lines which charac- 

 terize 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, correspond- 

 ing to different positions of the slit on the prominence, were re- 

 corded 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 eye- 

 piece 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 



