1v] NATURE OF THE RADIATIONS 127 
of Sidot’s hexagonal blend (phosphorescent zinc sulphide) lights 
up brightly under the action of the a rays of radium and polonium. 
Tf the surface of the screen is examined with a magnifying glass, 
the light from the screen is found not to be uniformly distributed 
but to consist of a number of scintillating points of ight. No two 
flashes succeed one another at the same point, but they are scattered 
over the surface, coming and going rapidly without any movement 
of translation. This remarkable action of the radium and polonium 
rays on a zine sulphide screen was discovered by Sir Wilham 
Crookest, and independently by Elster and Geitel?, who observed 
it with the rays given out from a wire which has been charged 
negatively either in the open air or in a vessel containing the 
emanation of thorium. 
In order to show the scintillations of radium on the screen, 
Sir William Crookes has devised a simple apparatus which he has 
called the “Spinthariscope.” A small piece of metal, which has 
been dipped in a radium solution, is fixed several millimetres away 
from a small zinc sulphide screen. This screen is fixed at one 
end of a short brass tube and is looked at through a lens fixed at 
the other end of the tube. Viewed in this way, the surface of the 
screen is seen as a dark background, dotted with brilliant points 
of light which come and go with great rapidity. The number of 
points of light per unit area to be seen at one time falls off rapidly 
as the distance from the radium increases, and, at several centi- 
metres distance, only an occasional one is seen. The experiment 
is extremely beautiful, and brings vividly before the observer the 
fact that the radium is shooting out a stream of projectiles, the 
impact of each of which on the screen is marked by a flash of light. 
The scintillating points of light on the screen are due to the 
impact of the @ particles on its surface. If the radium is covered 
with a layer of foil of sufficient thickness to absorb all the a rays 
the scintillations cease. There is still a phosphorescence to be 
observed on the screen due to the 8 and ¥ rays, but this luminosity 
is not marked by scintillations to any appreciable extent. Sir 
_ William Crookes showed that the number of scintillations was 
about the same in vacuo as in air at atmospheric pressure. If the 
1 Proc. Roy. Soc. 81, p. 405, 1903. 
2 Phys. Zeit. No. 15, p. 487, 1903. 
