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Dr. C. G. Barkla on 
ejected again along that axis. If we consider the radiating 
mass at the centre of a sphere with the direction of primary 
propagation as the axis, then the different possible planes of 
ejection are the planes through this axis, and as in each plane 
we may assume there is equal chance of ejection in all 
directions, the number of pairs received by any element of 
surface of the sphere is proportional to the density of the 
lines of longitude on that element. This varies as the secant 
of the latitude from 1 at the equator to infinity at the poles. 
The total energy of radiation received by a small area near 
the poles is therefore many times as great as that received by 
a similar small area near the equator ; that is, a small beam 
of secondary rays proceeding in a direction near that of 
propagation of the primary, is many times as intense as one 
in a direction approximately at right angles to this. 
To test between these theories, the simple apparatus was 
arranged as shown in the diagram. 
|A 2 „ 
A narrow pencil of X-rays emerged through a small 
circular aperture of "85 cm. radius in a lead box, and at a 
distance of 38 centimetres in this pencil a square slab of 
carbon (8x8 cm.) was placed in a vertical plane with its 
face equally inclined to the lines joining its centre to the 
centres of two equally distant rectangular apertures A 1 and 
A 2 in lead screens. Behind these were situated electroscopes 
