13 



which pass any given point P is propor- 

 tional to the defect of O P from T'O cm., 

 or in other words that the number // 

 which end their flight on any unit of 

 length of O N is a constant. The other 

 three groups of a particles have, as their 

 furthest distances of penetration, 4-8, 

 4*2, and 3'5 cm. respectively. Thus, 

 between 4*8 and 4 '2 In a particles end 

 their flight on each unit of length, be- 

 tween 4"2 and 3-5 the number is 3??, 

 and from that point up to the radium An. 

 The radium salt is supposed to be deep 

 enough to supply all these, i.e., its depth 

 is taken to be at least '002 cm. Suppose 

 now a powerful magnetic field to be 

 brought into play, the direction of the 

 lines of force being normal to the plane 

 of the paper. The paths of tlie a par- 

 ticles are curved to one side, and the 

 curvature is greater the nearer the 

 particle is to the end of its course. Let 

 O A and O Q represent two such paths. 

 Their separation from each other is con- 

 siderably exaggerated in the figure. If 

 all the paths were drawn the locus of Q 



would be seen to be a curve, whose curvature in contrast to 

 that of the path of any one particle would be greater the 

 further the distance from A. This is in agreement with M. 

 Becquerel's experiments, as I have previously pointed out."* 



The width of the trace upon the paper of all the paths 

 of the a particles is very small, and is almost too fine to be 

 shown on a diagram. 



It is perhaps well to point out that there is no break in 

 this trace at the critical points 4"8, 4'2, and 3*5. It is quite 

 smooth from end to end. These points mark the extreme 

 distance to which various bundles of a rays penetrate : but 

 the deflection of an a ray which ends its course at a given 

 point is independent of the particular radioactive material 

 from which it has come : the only varying characteristic of 

 an a particle is its velocity. 



We must now take into account that the widths of the 

 slit and the groove are not negligible, as is clearly to be 

 seen from the photograph under consideration. There is 



Fvg. 2 



* 'Thil. Mag.," December, 1904, p. 737. Jahrbuch d. Rad. n. 



Elektr., 1905, p. 14. 



