xxvii.] DISSYMMETRY. 385 



constitution of the two poles, but rather on some separation, is 

 rendered evident by the fact that on changing the direction of 

 the current, the calcium lines and some of the flutings in the 

 carbon spectra change position. 



So much for the spectrum of the poles themselves. 



If now we introduce a metal and observe its vapour, we find 

 a perfectly new set of phenomena. We get long and short 

 lines, but the law which they obey is no longer the one in 

 operation when the parts of the arc examined are symmetrical 

 with reference to the positive and negative poles, as when a 

 horizontal arc is employed. 



In my former work with a battery of thirty cells, in order to 

 obtain the lengths of the lines, it was necessary, in consequence 

 of the shortness of the arc, to throw an image of a horizontal 

 arc on the vertical slit of the spectroscope. In this manner 

 perfectly symmetrical photographs were obtained, the shortest 

 lines due to the core, and the middle portions of the longest ones, 

 lying in the axis of the photograph. 



In the plates obtained under the new conditions, the spectra 

 of those portions of the arc adjacent to the positive and negative 

 poles are widely dissimilar. Some lines stretch across the 

 spectrum with their intensities greatest close to one pole, 

 while other lines, invisible at this pole, are most intense at 

 the other. In one photograph, for instance, the Hue line of 

 calcium is visible alone at one pole, the H and K lines without 

 the Hue line, at the other. 



I have shown elsewhere that there is abundant evidence that 

 the H and K lines require a higher temperature to produce 

 them than that necessary for the production of the strong line 

 in the blue. 



More than this, there is a progression of lines, so to speak, 

 from pole to pole. They lie en echelon along the spectrum. 



In the case of other lines, only the central region of the arc 

 is occupied, the line being enormously distended there, either 



c c 



