708 Dr. F. W. Aston on a 



so that masses can be compared by measuring the ordinatesl 

 This method, though almost ideal for a general survey of: 

 masses and velocities, has objections as a method of precision, 

 many rays are lost by collision in the narrow canal-ray tube,, 

 the mean pressure in which must be at least half that in the 

 discharge-bulb ; very fine tubes silt np* by disintegration 

 under bombardment ; the total energy available for photo- 

 graphy falls off as the fourth power of the diameter of the 

 canal-ray tube. 



The first two can be overcome, as will be described below,, 

 by replacing the brass or copper tube by fine apertures made 

 in aluminium, a metal which appears to suffer no appreciable 

 disintegration, and by exhausting the space between these 

 apertures to the highest degree by means of a subsidiary 

 charcoal tube or pump. The falling off in intensity of the 

 parabolas as one attempts to make them finer is a very 

 serious difficulty, as the accuracy and resolving power depend 

 on the ratio of the thickness to the total magnetic deflexion ; 

 and if we increase the latter the electric deflexion must be 

 increased to correspond and the parabolas are drawn out,, 

 resulting again in loss of intensity. 



Methods of increasing the intensity of the spot. 



The concentration of the stream of positive rays down the 

 axis of the discharge-bulb is very marked, but there is good 

 evidence for assuming that the intense part of the stream 

 occupies a fairly considerable solid angle. This suggests the 

 possibility of an increase of intensity by means of a device 

 which should select the rays aimed at a particular spot on 

 the plate whatever direction they come from. For example,, 

 a thin gap between two coaxial equiangular cones would 

 allow the rays to be concentrated at the vertex. The 

 dimensions of the patch formed would be roughly those of 

 one given by a cylindrical canal-ray tube of diameter .equal 

 to the width of the gap. The increase of intensity would 

 therefore be considerable; but the method is not easy to put 

 into practice, and, in the case of deflexions through large 

 angles, would necessitate a curved photographic surface. 



Clearly the simplest way of increasing the intensity of the 

 spot without increasing its dimensions, at any rate in one 

 direction, is to use two parallel straight slits. In the case 

 of the method of crossed deflexions this device would only 

 be of use in a special case such as the resolution of a close 

 double, as the parabolas will only be sharp at points where 

 they are parallel to the slit. 



* L. c. p. 21 



