478 



Mr. W. Crookes on 



[Apr. 3, 



to excite phosphorescence. At a very high exhaustion there are fewer 

 collisions, and the initial speed of the molecules close to the negative 

 pole not being thereby reduced, phosphorescence takes place close to 

 the pole. 



Experiments are described in which a pole folded into corrugations 

 is used at one end of a tube, the pole at the other end being flat set 

 obliquely to the axis of the tube, and having a plate of mica in front 

 pierced with a hole opposite the centre of the pole. The questions which 

 this apparatus was designed to answer are: — (1.) Will there be two 

 sets of molecular projections from the corrugated pole when made 

 negative, one perpendicular to each facet, or will the projection be 

 perpendicular to the electrode as a whole, i.e., along the axis of the 

 tube ? (2.) Will the molecular rays from the oblique flat pole, when 

 this is made negative, issue through the aperture of the screen along 

 the axis of the tube, i.e., direct to the positive pole, or will they leave 

 the pole normal to the surface and strike the glass on its side ? 

 With the corrugated pole experiment shows that at high exhaustions 

 molecular rays are projected from each facet to the inner surface of 

 the tube, where they excite phosphorescence, and form portions of 

 ellipses by the intersection of the planes of molecular rays with the 

 cylindrical tube. When the oblique flat pole is made negative, a 

 stream of molecules shoots from it nearly normal to its surface, and 

 those which pass through the hole in the plate of mica strike the side 

 of the tube, forming an oval patch of a green colour. 



The oval patch in this apparatus happens to fall on a portion of the 

 glass which has previously had its phosphorescence excited by the 

 molecular discharge from the other corrugated pole. The phospho- 

 rescence from this pole is always more intense than that from the flat 

 pole, and the glass, after having been excited by the energetic bom- 

 bardment, ceases to respond readily to the more feeble excitement 

 from the flat pole. The effect, therefore, is, that when the oval spot 

 appears, it has a dark band across it where the phosphorescence from 

 the other pole had been taking place. The glass recovers its phos- 

 phorescent power to some extent after rest. 



In this apparatus a shifting of the line of molecular discharge is 

 noticed. If the coil is stopped and then set going repeatedly, always 

 keeping the oblique pole negative, the spot of green light occurs on 

 the glass at the spot where it should come supposing the discharge 

 were normal to the surface of the pole. But if once the flat pole is 

 made positive, the next time it is made negative the spot of light 

 appears nearer the axis of the tube, and instantly shifts to its normal 

 position, where it remains so long as its pole is made negative. There 

 seems no limit to the number of times this experiment can be repeated. 



A suggestion having been made by Professor Stokes that a third, 

 idle, pole should be introduced between the negative and positive elec- 



