McCuevyanp anp Hackerr—The Absorption of B Radium Rays by Matter. 48 
values of \’ as explained above. In measuring the energy of the radiation 
passing through any given plate, observations are taken with the ionisation 
vessel in the position (Z) represented in the diagram and also in a number of 
positions defined by different values of 6, the distance from the vessel to P 
being constant. 
The apparatus is so mounted that the vessel 7’ can readily be moved into 
these different positions. It is necessary to thus measure the radiation in 
different directions, because even when the incident pencil is chiefly in one 
definite direction, the portion of the issuing pencil that is composed of secondary 
rays has components in all directions. Even when there is no plate at P, it 
is necessary to take observations in different directions to get an accurate value 
of the energy of the incident pencil on account of the secondary rays from 
the walls of the opening through the lead screen. 
Having taken readings for the ionisation for a number of different values 
of 6, a curve is plotted having for coordinates 6 and the product of the reading 
by sin@. The area between the curve and the axis of @ is then proportional to 
the total radiation leaving the plate P. This follows since the area of the portion 
of the hemisphere between the cones of semi-vertical angles @ and 6 + d@ is 
proportional to sin #d@. Curves are plotted in this way corresponding to 
different thicknesses of plate P, and the areas between the curves and the axis 
of @ are taken as proportional to the total radiation that has passed through 
the plate. A value of \’ is then calculated from this total radiation with no 
plate at P, and the total radiation corresponding to each thickness of plate ; 
\’ is, of course, calculated from the equation above 
—Nd 
R, = he 0 
Having thus obtained several values of \’, we calculate the corresponding 
values of \ by using the equation (3) above, or rather another form of this 
equation more convenient for calculation. This coefficient \ is an accurate 
coefficient of absorption for all the radiation of primary and secondary 8 particles 
passing through the substance under examination. 
Finally, when desired, we can calculate the coefficient » from the equation 
N= =a 
This coefficient », as stated above, is the true coefficient of absorption of any 
given set of 8 particles; it is what the coefficient of absorption would be if 
there were no secondary 8 radiation. In all these calculations the values of 
x for the various substances are taken from the previous paper referred to 
above. 
