most easily observed. The experiment is an easy one to set up and to repeat. This property of "N" rays is analogous to that of the red and infra-red rays discovered by Edmond Becquerel. It is also analogous to the action of heat on phosphorescence. Nevertheless, I have not noticed as yet an increased rate of exhaustion of the phosphorescent capacity under the action of "N" rays (see p. 74).
The kinship of "N" rays with known radiations of large wave-length seems a certain fact. As, on the other hand, the property possessed by these rays of traversing metals differentiates them from all known radiations, it is very probable that they are comprised in the five octaves of the series of radiations, hitherto unexplored, between the Rubens rays and electro-magnetic oscillations of very small wave-length. This is what I propose to verify (note 8).