334 Mr 



Orange, On the shape of beams of canal-rays. 



On the shape of beams of canal-rays. By J. A. Orange, B.A., 

 Senior Scholar of Trinity College. (Communicated by Professor 

 Sir J. J. Thomson.) 



{Read 8 November 1909.] 



In a recent paper* the writer stated certain conclusions with 

 respect to beams of canal-rays obtained in connection with 

 " sandwich-kathodes." It was maintained that the canal-ra^/s 

 are straight, the curvature of the boundaries of the beam being 

 explained by supposing that they are merely the envelopes of 

 such rays. 



This point has been tested since by using a mica obstacle and 

 observing the form of the shadow in the beam. The method 

 of placing the obstacle will be evident from Fig. 1, where m is 

 the mica slip, which is attached to the aluminium kathode-plates 

 by means of water-glass. The beams of canal-rays were photo- - 



Fig. 1. 



graphed as described in the former paper. The sharpness of the 

 boundaries of the beams in the case of hydrogen was remarkable, 

 and a considerable degree of sharpness was obtained in the photo- 

 graphs, the softening that did occur being attributable to slight 

 changes in the form of the beams (due to changes of pressure) 

 while an exposure was being made. [The time of exposure was of 

 the order of 45 minutes.] Figs. 2 to 6 are copied from photographs 

 and illustrate the point at issue. 



The outlines of the beams and of the Crookes' dark space 

 at a fairly high pressure are shown in Fig. 2. For clearness' 

 sake, the shadows in the beams due to the three pins are omitted. 

 The beams will be referred to by the letters shown in Fig. 2. The 

 shadows due to the pins in Figs. 3 to 6 are in accordance with the 

 explanation adopted. 



With regard to the effect of the mica slip, it may be as well 

 to say at the outset that the use of this is liable to serious 

 objections. If the mica slip were an obstacle only to the canal- 

 rays, the method would be much sounder; but since the mica 

 is a local obstacle to the discharge in general, its introduction 



* J. A. Orange, Proc. Camb. Phil. Soc. Vol. xv. Pt. 3, 1909, p. 217. 



