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.