CH. vi.] DISCHARGE BY LIGHT 63 



There is no known way of actually observing this 

 quite invisible and purely theoretical trajectory ; but 

 when it is perceived that in accordance with this 

 theory all the particles moving between the plates 

 will have similar paths so far as they do not come 

 near the edge of either plate, in which case they 

 would not be propelled so far it becomes plain that 

 there should be a critical distance, within which the 

 gauze would receive and intercept all the particles, 

 and beyond which not a single one would be able to 

 reach it. In the figure the gauze is depicted as set 

 just beyond the critical distance, so that it would 

 receive no electricity, even though the ultra-violet 

 light were fully shining ; but so that if either its 

 distance from the zinc were diminished, or the electric 

 field strengthened, or the magnetic field weakened, 

 the gauze would at once come within range and 

 receive a plentiful supply of charge from the hypo- 

 thetical cycloidally-flying particles. And the critical 

 distance at which this would happen a thing easily 

 experimentally observed would be independent of 

 the brightness of the ultra-violet light, and would 

 merely equal the diameter of the generating circle. In 

 other words, the critical distance between the plates, 

 when effective transfer of charge occurred, should be 



2a, or ; a quantity which by this ingenious 



6-Ll 



means is therefore measurable. Wherefore the ratio 

 m/e for this case can be experimentally determined, 

 if E and H are both known. The apparatus 

 employed was shown in Fig. 8. 



The sharpness of actual experimental observation 

 of the critical distance was not found quite so great 

 as this simple theory would indicate, because of 

 disturbing causes ; one of which was the presence of 



