The essential property which allows them to be revealed is their action on a small induction spark, of which they increase the brilliancy; this phenomenon is visible to the eye and is rendered objective by photography.
Various other physicists and numbers of physiologists, following the path opened by M.Blondlot, published during 1903 and 1904 manifold but often rather hasty memoirs, in which they related the results of their researches, which do not appear to have been always conducted with the accuracy desirable.

These results were most strange; they seemed destined to revolutionise whole regions not only of the domain of physics, but likewise of the biological sciences.

Unfortunately the method of observation was always founded on the variations in visibility of the spark or of a phosphorescent substance, and it soon became manifest that these variations were not perceptible to all eyes.
No foreign experimenter has succeeded in repeating the experiments, while in France many physicists have failed; and hence the question has much agitated public opinion.

Are we face to face with a very singular case of suggestion, or is special training and particular dispositions required to make the phenomenon apparent?
It is not possible, at the present moment, to declare the problem solved; but very recent experiments by M.Gutton and a note by M.Mascart have reanimated the confidence of those who hoped that such a scholar as M.
Blondlot could not have been deluded by appearances.

However, these last proofs in favour of the existence of the rays have themselves been contested, and have not succeeded in bringing conviction to everyone.
It seems very probable indeed that certain of the most singular conclusions arrived at by certain authors on the subject will lapse into deserved oblivion.