Rajs of Positive Electricity. 755 



velocity of the rays producing - it is independent of the 

 potential difference between the electrodes, finally when the 

 pressure is very low it looks like a faint nebulous band over 

 which brighter patches are superposed. 



The relations between the positive and negative portions of 

 the phosphorescent figures when the pressure is low are 

 very interesting. The lower, more deflected portion has 

 frequently two bright spots A and B for each of which 

 e/m= 10* : one at A which gradually moves, as the pressure 

 is diminished, along a parabolic path to 0, the position of the 

 undetected spot ; the other, not quite so definite, at B, a point 

 on the phosphorescent band which has survived from the 

 higher pressure. The negative portion at these low pressures 

 is not a replica of the positive portion as it was at the higher 

 pressures, but remains unaltered in shape and position as the 

 pressure diminishes, getting gradually fainter. There is no 

 trace on it of the spot A ; the spot B is, however, visible at B', 

 and the luminous band BB' can be traced as a straight strip 

 occupying the same position as it did at higher pressures 

 when it was the only part of the phosphorescence visible. 



There is nothing on the negative side corresponding to the 

 portion OCD on the positive, or at any rate if it exists it is 

 so very much fainter, that I have never been able to satisfy 

 myself of its existence, even when the negative part OB' was 

 quite bright. 



I think there is exceedingly strong evidence to show that 

 the straight band of phosphorescence which alone is seen at 

 higher pressure and which lingers on with diminished intensity 

 when the pressure is reduced, has a different origin from the 

 phosphorescence which shows itself as bright spots on an 

 isolated streak of phosphorescence, and which- is due to rays 

 whose velocity, unlike that of those producing the first kind 

 of phosphorescence, depends upon the potential difference 

 between the electrodes. 



Such evidence is afforded by the following experiments, 

 the first of which shows the complete symmetry between the 

 positive and negative parts of the first kind of phosphorescence, 

 and also that much of this kind of phosphorescence is due to 

 secondary rays produced after the primary rays have passed 

 through the cathode. In this experiment, the magnetic and 

 electric fields, instead of being as in the previous experiments 

 arranged so that when a particle was exposed to a magnetic 

 force it was simultaneously exposed to an electric one, were 

 made to overlap. The poles MM of the electromagnet were 

 pushed nearer the screen so that they extended on the screen 

 side beyond the parallel plates PP which produced the electric 



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