18 THE CHEMISTRY OF THE SUN. [CHAP. 



will consequently become indistinct, and disappear when the 

 aperture is too great. 



He was not content with this work. He wanted to know 

 something of the origin of the lines, and he soon came to a 

 conclusion on this point. It occurred to him that they might 

 possibly be attributed to some illusion caused by the narrow 

 aperture through which the light was admitted. We shall 

 see subsequently that the slit has something to do with the 

 forms of these dark spaces, but with their simple existence 

 as spaces it has nothing to do, the mere shape of the lines 

 being quite a trivial matter. To test this he had recourse 

 to a very ingenious method, which is best described in his 

 own words : l 



" In observing the great quantity of lines in the solar spectrum 

 we might be led to believe that the inflection of light at the narrow 

 aperture in the window-shutter had some connection with them, 

 though the experiments described do not give the least proof of 

 this, and indeed establish the contrary opinion. In order to put 

 this beyond a doubt, and also to make some other observations, I 

 varied the experiments in the following manner : if we make the 

 sun's rays pass through a small round aperture in the window- 

 shutter, nearly 15" in diameter, and cause it to fall on a prism 

 placed before the telescope of the theodolite, it is obvious that the 

 spectrum seen by the telescope can only have a very small width, 

 and consequently will form only a line. In a line, however, 

 almost no breadth it is impossible to see the fine and delicate lines 

 which traverse it ; and, on that account, the fixed lines are not seen 

 in a spectrum of this kind. In order, however, to see all the lines 

 in this spectrum, it is necessary only to widen it by an object-glass, 

 without altering its length. I obtained this effect by placing 

 against the object-glass a glass having one of its faces perfectly 

 plane, and the other ground into the segment of a cylinder of a 

 very great diameter. The axis of the cylinder was exactly parallel 

 to the base of the prism. . . ." 



1 E<lin. Phil. Journal, vol. x. pp. 37 and 38. 



