See Turpain, _Lecons elementaires de
Physique_, t. ii. p. 311, for details of the experiment.--ED.]
M.H. Poincare has pointed out, however, that we know nothing as to the
mechanism of the photographic impression. We cannot consider it
evident that it is the kinetic energy of the ether which produces the
decomposition of the sensitive salt; and if, on the contrary, we
suppose it to be due to the potential energy, all the conclusions are
reversed, and Neumann's idea triumphs.
Recently a very clever physicist, M. Cotton, especially known for his
skilful researches in the domain of optics, has taken up anew the
study of stationary waves. He has made very precise quantitative
experiments, and has demonstrated, in his turn, that it is impossible,
even with spherical waves, to succeed in determining on which of the
two vectors which have to be regarded in all theories of light on the
subject of polarization phenomena the luminous intensity and the
chemical action really depend. This question, therefore, no longer
exists for those physicists who admit that luminous vibrations are
electrical oscillations.
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