We then perceive multitudes of
brilliant points on the screen, which appear and at once disappear,
producing a scintillating effect. It seems probable that every
particle falling on the screen produces by its impact a disturbance in
the neighbouring region, and it is this disturbance which the eye
perceives as a luminous point. Thus, says Sir W. Crookes, each drop of
rain falling on the surface of still water is not perceived as a drop
of rain, but by reason of the slight splash which it causes at the
moment of impact, and which is manifested by ridges and waves
spreading themselves in circles.
The various radioactive substances do not all give radiations of
identical constitution. Radium and thorium possess in somewhat large
proportions the three kinds of rays, and it is the same with actinium.
Polonium contains especially alpha rays and a few gamma rays.[35] In
the case of uranium, the alpha rays have extremely slight penetrating
power, and cannot even impress photographic plates. But the widest
difference between the substances proceeds from the emanation.
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