in general show a diffuse border round the shadow of the object. For this reason radiographs of this kind lack the sharpness of outline of X ray photographs.
110. Secondary radiation produced by α rays. Mme Curie68 has shown by the electric method that the α rays of polonium produce secondary rays. The method adopted was to compare the ionization current between two parallel plates, when two screens of different material, placed over the polonium, were interchanged.
These results show that the α rays of polonium are modified in passing through matter, and that the amount of secondary rays set up varies with screens of different material. Mme Curie, using the same method, was unable to observe any such effect for the β rays of radium. The production of secondary rays by the β rays of radium is, however, readily shown by the photographic method. We have already seen (section 93) that very low velocity electrons accompany the α rays from radium or radio-tellurium spread on a metal plate. These electrons are probably liberated when the α rays escape from or impinge upon matter, and the number emitted depends upon the kind of matter used as a screen. The differences shown in the above table when the screens were