The radiations of very small wave-length, discovered by M. Schumann, are to a very great extent absorbed by air; "N" rays are not. This implies the existence of absorption bands between the ultra-violet spectrum and "N" rays. The wave-length of "N" rays increases with their refractive index, contrary to what occurs with luminous radiations.

If the increase in brilliancy of a small luminous source by the action of "N" rays is to be attributed to a transformation of these radiations into luminous radiations, this transformation is in conformity with Stokes' law.

Registration by Photography of the Action produced by "N" Rays on a small Electric Spark (February 22, 1904).

Though "N" "N" rays have no intrinsic action on the photographic plate, it is nevertheless possible to utilize photography to reveal their presence and study their action. This object is attained, as I showed as long ago as May II, 1903, by making a small, luminous source

act for a determined period on a sensitive plate, whilst this source is subjected to the action of “N” rays, and then repeating the experiment for the same interval of time and under the same conditions, save that the "N" rays are suppressed. The impression produced is notably more intense in the first case than in the second. As an example of the application of this method, I gave at the time two photoengravings, whose comparison shows that water, even when used in very thin films, arrests "N" rays issuing from an Auer burner (see page 16). Since then I have extended the experiments to the registration of actions produced by "N" rays from various sources, and I have perfected the process, as will be shown.

A small, luminous spark is the most appropriate luminous source for this kind of investigation for, on the one hand, it is very actinic, and, on the other, it can be maintained as long as necessary at the same intensity. Although it is impossible to obtain absolute steadiness of glow in the spark, since these variations are not produced systematically,

their influence should disappear in the total impression received by the plate, even after a very short exposure. I contrived, besides, to eliminate even still more completely this cause of perturbation, by repeatedly alternating the experiments, as I will proceed to show.

Fig. 4 represents a horizontal section of the apparatus employed. AB is the photographic plate, 13 cms. wide; E is the spark enclosed in a cardboard box, FGHI, open only on the side facing the plate, and allowing the spark to act on one half, OB, of the plate only; CD is a lead screen wrapped in wet paper, rigidly connected with the frame which holds the plate. The "N" rays, proceeding from any source, form a pencil, having the direction NN'. With this arrangement the "N" rays are arrested by the screen CD; the spark, while it acts on half-plate OB, is sheltered from the rays.

Now impart to the frame containing the plate a translation to the right equal to half its length (Fig. 5); the other half, AO, of the plate takes the place formerly occupied by