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several minutes, this face alone shows activity; an exposure of several hours is necessary for the activity to reach the opposite face.

Aluminium, wood, dry or wet paper, and paraffin do not enjoy the property of storing "N" rays. Calcium sulphide, on the other hand, does possess this property. When I put a few grams of sulphide in an envelope, and then exposed the envelope to "N" rays, I found that its proximity was sufficient to reinforce the phosphorescence of a small mass of previously excited sulphide. This property explains a constant peculiarity that I have previously set forth, viz. that the increase of phosphorescence under the action of "N rays takes an appreciable time whether to appear or to disappear. For, thanks to the storing-up of the "N" rays, the different parts of a mass of sulphide mutually reinforce their phosphorescence; but since, on the one hand, this reinforcing is progressive, as I have directly proved, and since, on the other hand, the stored-up provision is not immediately exhausted, the result is that when "N" rays are made to fall on phosphorescent calcium

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sulphide, their effect must increase slowly, and that when they are suppressed, their effect can only disappear slowly.1

Pebbles picked up at about four o'clock p.m., in a yard where they had been exposed to the sun, spontaneously emitted "N" rays; bringing them near a small mass of phosphorescent sulphide was sufficient to increase its luminosity. Fragments of calcareous stone, brick, etc., picked up in the same yard, produced analogous actions.

The activity of all these bodies still persisted after four days, without any sensible diminution. It is, however, necessary for the manifestation of such actions that the surface of these bodies should be quite dry; for we know that the thinnest layer of moisture is sufficient to arrest "N" rays. Vegetable earth was found to be inactive, doubtless on account of its moisture; pebbles taken from several centimetres underneath the surface of the soil were inactive, even after being dried.

1 I repeat here that, as a rule, when experimenting with "N" rays, it is advantageous to replace the Auer burner by a Nernst lamp absorbing about 200 watts.

The phenomena of the storing-up of "N" rays, which are the object of the present note, ought naturally to be compared with those of phosphorescence; yet they present a quite distinct feature, as I intend to show shortly.

On the Strengthening Action of a Beam of Light on the Eyes, when the Beam is accompanied by "N" Rays (November 23, 1903).

While studying the storing-up of "N" rays by different bodies, I had occasion to observe an unexpected phenomenon. My eyes were fixed on a small slip of paper, dimly lighted, distant about 1 metre from me; a brick, one of whose faces had been sun-exposed, having been brought near laterally to the luminous pencil, with its sun-exposed face turned towards me, and a few decimetres distant from my eyes, I saw the slip assume a heightened glow; when the brick was removed, or when its non-exposed face was turned towards me, the paper grew

darker. To remove all possibility of illusion, I arranged permanently a box closed by a cover and wrapped in black paper; in this completely enclosed box the brick was placed, and, in this manner, the dark background on which the slip stood out remained rigorously invariable, but the observed effect remained the same. The experiment can be varied in different ways. For instance, the laboratory shutters being almost closed, and the dial of the clock fixed to a wall which was just sufficiently lighted for the dial, at a distance of 4 metres, to be just perceived as a grey patch with no defined contour, if the observer, without changing his place, directs towards his eyes the "N" rays emitted by a previously exposed brick or pebble, he sees the dial whiten; he can trace distinctly its circular contour, and even succeed in seeing the hands. When the "N" rays are suppressed, the dial again grows dark. Neither the production nor the cessation of the phenomenon are instantaneous.

As in these experiments the luminous object is placed very far away from the source of "N rays, and as, on the other hand, in order that

the experiment may succeed, the rays must be directed, not towards the object, but towards the eye, there can be no question here of an increase in emission of a luminous body influenced by "N" rays, but indeed of a strengthening of the effect upon the eye, due to the "N" rays which are superposed on the luminous rays.

This fact astonished me all the more because, since the slightest film of water arrests "N" rays, it seemed unlikely that they could penetrate into the eye, whose humours contain more than 98.6 per cent. of water (Lohmeyer). The small quantity of salt contained in these humours must have rendered them transparent to "N" rays. But, then, in all probability, salt water must itself be transparent. Experiment shows that this is the case, for while a sheet of wet paper completely arrests "N" rays, a vase of Bohemian glass, 4 cms. in diameter, filled with salt water and placed in their path, lets them pass without sensible weakening. A very small quantity of sodium chloride is sufficient to render water transparent. What is more, salt water is capable of storing-up "N" rays, and

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