Another resemblance results also from the experiments by which M.Perreau established that these rays act on the electric resistance of selenium.

New and valuable arguments have thus added force to those who incline towards a theory which has the merit of bringing a new phenomenon within the pale of phenomena previously known.
Nevertheless the shortest ultra-violet radiations, such as those of M.
Schumann, are still capable of refraction by quartz, and this difference constitutes, in the minds of many physicists, a serious enough reason to decide them to reject the more simple hypothesis.
Moreover, the rays of Schumann are, as we have seen, extraordinarily absorbable,--so much so that they have to be observed in a vacuum.

The most striking property of the X rays is, on the contrary, the facility with which they pass through obstacles, and it is impossible not to attach considerable importance to such a difference.
Some attribute this marvellous radiation to longitudinal vibrations, which, as M.Duhem has shown, would be propagated in dielectric media with a speed equal to that of light.

But the most generally accepted idea is the one formulated from the first by Sir George Stokes and followed up by Professor Wiechert.

According to this theory the X rays should be due to a succession of independent pulsations of the ether, starting from the points where the molecules projected by the cathode of the Crookes tube meet the anticathode.