PLEOCHROIC HALOES—JOLY. 315 
millimeters of the range where the rate of ionization and of the speed 
of the ray change most rapidly. To what are the changing properties 
of the rays near the end of their path to be ascribed? It is only 
recently that this matter has been elucidated. 
When the alpha ray has sufficiently slowed down, its power of 
passing right through atoms, without appreciably experiencing any 
effects from them, diminishes. The opposing atoms begin to exert 
an influence on the path of the ray, deflecting it a little. The heavier 
atoms will deflect it most. This effect has been very successfully 
investigated by Geiger. It is known as “‘scattering.’’ 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 toward the end of its 
path. In this case the path of 
the ray has been rendered visi- 
ble by the condensation of water 
particles under the influence of 
the ionization; the atmosphere 
in which the ray travels being in 
a state of supersaturation with 
water vapor at the instant of 
the passage of the ray. It is 
evident that if we were observing the ionization along a sheaf of 
parallel rays, all starting with equal velocity, the effect of the bend- 
ing of some of the rays near the end of their range must be to cause 
a decrease in the aggregate ionization near the very end of the ulti- 
mate range. For, in fact, some of the rays complete their work of 
ionizing 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 
ionization near the end of the range, when the effects of a bundle of 
rays are being observed. The explanation does not suggest that the 
ionizing power of any one ray is actually diminished before it finally 
ceases to be an alpha ray.’ 
The full line in figure 1 gives the ionization curve which it may be 
expected would be struck out by a single alpha ray. In it the 
lonization goes on increasing till it abruptly ceases altogether, with 
the entire loss of the initial kinetic energy of the particle. 
Nae 
\ 
Fig. 2. 
