"N" Rays/On the Photographic Registering of the Action Produced by "N" Rays on a Small Electric Spark
Registration by Photography of the Action produced by "N" Rays on a small Electric Spark (February 22, 1904).
Though "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 11, 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 photo-engravings, 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, 
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 OB; and this time the screen CD, carried along with the frame in this movement, is no longer interposed in the path of the rays. The half-plate AO therefore receives the action of the spark while subjected to the rays.
This being understood, the experiment is as follows: first the plate is kept in the first of the above-indicated positions during five seconds, then in the second position also for five seconds; it is then brought back to the first position, and the double operation just described is repeated several times.
After an interval of time equal to an even multiple of five seconds—for instance, one hundred seconds—each of the half-plates has been exposed to the spark for an equal period, only, while AO was exposed, "N" rays were in action, and while OB was exposed there were none.
Thanks to an arrangement of guides and buffer-stops, the to-and-fro motion of the frame can be executed with perfect certainty and regularity, in spite of the darkness. A metronome is used to regulate the action.
The spark is produced by a small induction-coil, known as du Bois-Reymond's chariot apparatus; it strikes between two blunt points of platinum-iridium, carefully machined and polished. These are fixed to the two jaws of a pair of wooden pliers, which tend to close by elasticity, and are kept apart by a micrometer screw. At a distance of about 2 cms. from the spark, and facing the plate, a plate of ground glass is fixed. As I have previously mentioned, the light of the spark produces on this glass an extensive luminous patch, much easier to observe than the naked spark, and giving on the photographic plate impressions of much more regular form. The regulating of the spark is the delicate part of the experiment. The induced current must first be adjusted, by modifying the primary current on the one hand, and the position of the secondary coil on the other, till the spark becomes very small. The points are washed in alcohol, then a slip of dry paper is drawn between them, for the purpose of drying and repolishing their surface; then the micrometer screw is turned so as to make the spark as short as possible, yet without incurring any risk of the points touching by any chance vibration, which would make it disappear intermittently. By a methodical process of trial and error, which sometimes demands much time and patience, one succeeds in getting a spark both regular and very feeble; it is then sensitive to the action of "N" rays. If one directs on it a pencil of these radiations, proceeding from any source, one sees the patch on the ground glass increase in size and glow; at the same time its central part becomes more luminous, appearing wrapped in a kind of nimbus. One can then proceed with the photographic experiment. I made about forty such experiments, employing in turn, as sources of "N" rays, a Nernst lamp, compressed wood, hardened steel, Rupert's drops, etc. I have varied the experiments in different ways—for example, by changing the side of the screen CD, by using a zinc screen transparent to "N" rays, etc. Several eminent physicists, who have been good enough to visit my laboratory, have witnessed them. Of these forty experiments, one was unsuccessful: the rays were produced by a Nernst lamp, and instead of 
the expected unequal impressions, two sensibly identical images were obtained. I believe this failure, unique, be it remarked, to be due to an insufficient regulation of the spark, which, doubtless, was not sensitive. Fig. 6 is a photo-engraved reproduction of the prints obtained with and without "N" rays issuing from a Nernst lamp.
Fig. 7, similarly, shows the result of an experiment with "N" rays, produced by two large files.
Though the photogravures are far from rendering in a satisfactory manner the aspect of the originals, they nevertheless show the influence of "N" rays on a photographic impression.
I give further (Figs. 8 and 9) the reproduction of photographs, showing that "N" rays, issuing from a Crooke's tube, are polarized.
These photographs date from the month of April, 1903. They were not obtained by the method of reiterated alternation of exposure, as this method is difficult to apply to this case; but the experiments have been repeated a great number of times with the most minute precautions, and the constancy of the results is an absolute guarantee of their worth.
From my communication of May 11, 1903, and from what precedes, it is clear that, from the beginning of my researches on "N" rays, I had succeeded in recording their action on the spark by an objective method.