141) and Professor Soddy (_Radio-Activity_, London, 1904, p.

66).

Neither Mr Whetham, in his Recent _Development of Physical Science_ (London, 1904) nor the Hon.

R.J.Strutt in _The Becquerel Rays_ (London, same date), both of whom deal with the historical side of the subject, seem to have noticed the fact .-- ED.] To these radiations there sometimes are added in the course of experiments secondary radiations analogous to those of M.Sagnac, and produced when the alpha, beta, or gamma rays meet various substances.
This complication has often led to some errors of observation.
Phosphorescence and fluorescence seem especially to result from the alpha and beta rays, particularly from the alpha rays, to which belongs the most important part of the total energy of the radiation.
Sir W.Crookes has invented a curious little apparatus, the spinthariscope, which enables us to examine the phosphorescence of the blende excited by these rays.

By means of a magnifying glass, a screen covered with sulphide of zinc is kept under observation, and in front of it is disposed, at a distance of about half a millimetre, a fragment of some salt of radium.