298 
Transact 1 071^ of the Royal Society of Smdli Africa. 
air* lying between 1 x 10^" and 2 x XO^*^. These numbers fix the dimensions 
of the apparatus needed, and fig. 1 represents the first arrangement adopted 
in order to secure a direct /i-particle photograph. A "slit" of Rontgen- 
rays \ h mm. broad coming directly from the tungsten anticathode of a 
Coolidge tu])e fell upon the red-leaded surface of the wax slab ab placed at 
an angle of G° to the surface of the photographic plate cd, the beam of 
Rontgen-rays ])eing carefully " sighted " in a preliminary experiment so as 
to ensure that it did not touch the photographic plate, but ran parallel to its 
surface at a distance of about 0 2 cm. from it. At x and y the red-lead 
was removed from the wax along two ])road lines. ^J'his was done as a 
control, for paraffin wax emits no /^-particles. The plates used were " Imperial 
Special Rapid," the exposure was for five minutes with 7 amperes in the 
primary of a Newton-Apps 10-in. coil, the tube backing up a parallel spark 3 in. 
in length between balls. The resulting photograph is shown in register in fig. 2, 
and shows unmistakably that the impression has come from the red-lead film 
opposite. These photographs are positives, a darkening representing absence 
of photographic action. The line y is not clearly seen because it was much 
narrower than x, the diffusion of the particles from surrounding regions 
rendering it invisible, and it probably fell on the edge of the incident beam. 
In order to find how much of the effect is due to the diffusing /^-particles 
or to the secondary Rontgen-rays from the red-lead, a cross of paraffin wax 
0-005 cm. in thickness and one of mica 0-00136 cm. in thickness were stuck 
on to the photographic plate with a touch of glue (shown by the black spots 
in the middle of the crosses, fig. 3) in the region directly opposite to that 
part of the red-lead surface acted upon by the Rontgen-rays. The resulting 
photograph shows in a striking manner that of the two influences it is the 
/^-radiation that produces the photographic impression, the photographic 
plate being effectively shielded by so small a mass of substance, far too 
small to be effective in stopping a Rontgen-ray beam properly so-called. 
The red-lead surface was about 0*35 cm. above the wax cross and 0-5 cm. 
above the mica one. 
A preliminary experiment was now performed in order to determine the 
maximum range at which these particles could produce a photographic 
impression. Using the same arrangement as above, the angle between the 
slab and the photographic plate was doubled, and the whole system shifted 
laterally with respect to the incident l)eam of Rontgen-rays by intervals of 
1 mm., allowing a 3-minute exposure at each shift. In this manner 
practically the whole length of the red -leaded surface was exposed to the 
rays. In order to show up any effect on the photographic plate opposite, 
this was covered by a perforated strip of tissue paper 0-0034 cm. thick, 
absorbinn' no Rontgen-rays falling on the plate but effectively stopping any 
/i-particles. On the negative itself, fig. 4, the strip is visible right to the 
* Boln- gives 11 x 10^^' Phil. Mag.' (6), xxv, p. 28, 1913. Whiddington 2-1 x 10*o, 
' Proc. Eoy. Soc' (a), Ixxxvi (1912), p. 370. 
