CONTEMPORARY ADVANCES IN PHYSICS 



649 



sloping section, adjoined on the right by a steeper descent to the axis of 

 abscissae — the sign of a "group" of protons comparatively fast. This 

 descent occurred at smaller values of range, the greater the angle 0. 

 This implied that the mean speed of the group depends on 9 — the 

 mechanism for ejecting these protons functions in such a way as to 

 give less energy to those which fly off more obliciuely. Pose extended 

 his own researches on aluminium and obtained curves corresponding 

 to the topmost of Fig. 6, for various values of 9 ranging up to 135°. At 

 this last cited angle, the ranges of the three groups had sunk from 57 

 and 48 and 28 to 45 and 38 and 20 respectively.^ 



20 

 CM. AIR 



25 



Fig. 8 — -Integral distribution-in-range curves for protons emitted from disrupted 

 nuclei of boron and other atoms (W. Bothe and H. Franz). 



Whatever this fact may mean in regard to the mechanism, its prac- 

 tical consequence is clear. If the speeds of the protons depend on their 

 direction of departure, then the interval over which these speeds are 

 distributed for any given direction can be appreciated in its true nar- 

 rowness (whatever that may be) solely by observing the protons which 

 come off in that direction only. If in the actual experiment the paths 

 of the alpha-particles falling upon the bombarded substance diverge 

 over a wide solid angle, and if the paths of the protons which the 

 counter or the fluorescent screen receives diverge likewise over a wide 

 solid angle, then the sharpness of the groups must necessarily be 

 masked. Now of course one would desire in any case to reduce these 



4 Before the discovery of groups, it had been observed at Cambridge that the 

 maximum range of protons projected straight forward is greater than the maximum 

 found among those projected almost straight backward: for boron^the two values 

 were 58 and 38, for aluminium 90 and 67 (in air at 760 mm. and 15° C.) 



