388 Messrs. W. Spottiswoode and J. F. Moulton. [Jane 16, 



IX. " On Stratified Discharges. VII. Multiple Radiations from 

 the Negative Terminal." By William Spottiswoode, 

 P.R.S., and J. Fletcher Moulton, F.R.S. Received 

 June 1(5, 1881. 



In our memoirs " On the Sensitive State of Vacuum Discharges " 

 we have often alluded to, and even insisted on, the importance of the 

 remarkable dissymmetry which manifests itself in electrical discharges 

 in gases at low pressures. As the pressure is lowered this dissymmetry 

 becomes more and more marked ; the striae themselves become indi- 

 vidually unsymmetrical, and recede one by one into the positive ter- 

 minal ; the features which are associated with the negative increase 

 in importance, until at last they occupy almost the entire area of the 

 tube. The researches of De La Rue, Crook es, Goldstein, and others, 

 have intended to increase the interest which attaches to the action 

 which takes place at the negative terminal. And it is with a view of 

 adding one more contribution to the interpretation of the phenomena 

 of the negative discharge and the analysis of its nature and modus 

 operandi that the present experiments are described. 



On examining the image of a negative terminal as traced out in 

 tubes of great exhaustion, by the phosphorescence due to Crookes' 

 radiations, we have often noticed that the image was not a simple 

 figure, but that more than one outline of the contour of the terminal 

 might be traced. From the fact of the double contour having been 

 first remarked, where the terminal was of a conical form, it was at 

 first supposed that the second image might be due to internal reflexion, 

 or to some property appertaining to the edge of the cone. But this 

 supposition led to no satisfactory explanation of the phenomenon. 



It was, however, thought that, inasmuch as the two images implied 

 different systems of radiation, a magnet, suitably disposed, might 

 affect them in different degrees, and thereby throw some light on 

 their origin. For this purpose we used a large electro-magnet with 

 its coils so coupled up as to give the two poles similar polarity. By 

 bringing the two poles together, inclined at a moderate angle, a single 

 pole and a field of great magnetic strength was produced. 



The tube was then placed in the plane containing the axis of the 

 two poles, and in the direction of a line bisecting their directions. 

 The tubes first used were of great exhaustion, and were placed some- 

 times with the positive, sometimes with the negative terminal towards 

 the magnet. When the tube was placed in a comparatively weak 

 part of the field, the two images of the cone were seen in their usual 

 positions relatively to each other, except that they were slightly more 

 separated. But as the tube was brought gradually into a stronger 

 part of the field, the two images became further separated, and by 



