PLEOCHROIC HALOES 133 



exert an influence on the path of the ray, deflect- 

 ing it a little. The heavier atoms will deflect it 

 most. This effect has been very successfully in- 

 vestigated by Geiger. It is known as " scatter- 

 ing." The angle of scattering increases rapidly 

 with the decrease of velocity. Now the effect of 

 the scattering will be to cause some of the rays 

 to complete their ranges or, more accurately, to 

 leave their direct line of advance a little sooner 

 than others. In the beautiful experiments of 

 C. T. R. Wilson we are enabled to obtain ocular 

 demonstration of the scattering. The photograph 

 (Fig. 2), which I owe to the kindness of Mr. 

 Wilson, shows the deflection of the ray towards 

 the end of its path. In this case the path of the 

 ray has been rendered visible by the condensation 

 of water particles under the influence of the 

 ionisation; the atmosphere in which the ray 

 travels being in a state of supersaturation with 

 water vapour at the instant of the passage of the 

 ray. It is evident that if we were observing the 

 ionisation along a sheaf of parallel rays, all start- 

 ing with equal velocity, the effect of the bending 

 of some of the rays near the end of their range 

 must be to cause a decrease in the aggregate 

 ionisation near the very end of the ultimate range. 

 For, in fact, some of the rays complete their work 

 of ionising at points in the gas before the end is 

 reached. This is the cause, or at least an 

 important contributory cause, of the decline in 

 the ionisation near the end of the range, when the 



