120 NATURE OF THE RADIATIONS [CH. 
be completely deviated by the magnetic field. (4) comprises the 
effect of the y rays together with the natural leak of the electro- 
scope in hydrogen. 
In this experiment there was a good deal of stray magnetic 
field acting on the rays before they reached the pole-pieces. The 
diminution of the rate of discharge due to the a rays was found to 
be proportional to the strength of field between the pole-pieces. 
With a more powerful magnetic field, the whole of the a rays were 
deviated, showing that they consisted entirely of projected charged 
particles. 
In order to determine the direction of deviation of the rays, 
the rays were passed through slits one mm. in width, each of which 
was half covered with a brass strip. The diminution of the rate of 
discharge in the testing vessel for a given magnetic field in such a 
case depends upon the direction of the field. In this way it was 
found that the rays were deviated in the opposite sense to the 
cathode rays. Since the latter consist of negatively charged 
particles, the a rays must consist of positively charged particles. 
These results were soon after confirmed by Becquerel?, by the 
photographic method, which is very well adapted to determine the 
character of the path of the rays acted on by a magnetic field. 
The radium was placed in a linear groove cut in a small block of 
lead. Above this source, at a distance of about 1 centimetre, was 
placed a metallic screen, formed of two plates, leaving between them 
a narrow opening paralled to the groove. Above this was placed 
the photographic plate. The whole apparatus was placed in a 
strong magnetic field parallel to the groove. The strength of the 
magnetic field was sufficient to reflect the 8 rays completely away 
from the plate. When the plate was parallel to the opening, 
there was produced on it an impression, due to the a rays alone, 
which became more and more diffuse as the distance from the 
opening increased. This distance should not exceed 1 or 2 centi- 
metres on account of the absorption of the rays im air. If, during 
the exposure, the magnetic field is reversed for equal lengths of 
time, on developing the plate two images of the a rays are 
observed which are deflected in opposite directions. This devia- 
tion, even in a strong field, is small though quite appreciable and 
1 C. R. 136, p. 199, 1903. 
