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the abscissæ stand for the wave-lengths and the ordinates for the indices diminished by unity.
Each of the divisions marked on the axis of abscissæ corresponds to O‘001, and each of the divisions marked on the ordinate axis corresponds to an excess over unity equal to o‘ol.
In spite of all the care with which the experiments were executed, the deviations are so small, and, consequently, the indices so near to unity, that the table and diagram can only be regarded as a preliminary indication of the behaviour of the dispersion in the very slightly deviated part of the spectrum. An important consequence arises from these measures, viz. points corresponding to “N” rays, and those corresponding to N, rays, are all situate on the same curve, within the limits of experimental error. The study of radiations still less refrangible than those I have dwelt on appeared to me impracticable. To avoid confusion, I was obliged to adopt a very large scale for the ordinates ; this is why I could not plot on the diagram the results of my former measurements of the more refrangible “N” rays (loc. cit.). These results give points situated on a branch of the curve, starting from the topmost point on the right, and rising almost vertically, with a feeble inclination, from bottom to top, and from right to left, and a slight convexity turned upwards.
Certain sources seem to emit N, rays exclusively, or, at least, these rays predominate in the emission. This is the case with copper and silver wire, and with hard-drawn platinum wire. M. Bichat has observed that ethylic ether, when brought to the state of forced extension, by the process discovered by M. Berthelot, emits N, rays. When this state of strain ceases, whether spontaneously or under the action of a slight blow, the emission of N, rays immediately disappears.
N, rays can be stored up like “N” rays. For instance, one need only bring a bit of stretched copper wire in proximity to a lump of quartz to make the quartz emit N, rays for some time after.
On Peculiarities presented by the Action
exercised by “N” Rays on a Dimly Lighted Surface (February 2, 1904).
Consider a phosphorescent screen, or, more generally, a dimly lighted surface. If this surface is viewed normally, one notices that the action of “N” rays is to render it more luminous ; if, on the contrary, the surface is viewed very obliquely, nearly tangentially, the action of “N” rays is to render it less luminous. In other words, the action of “N” rays increases the quantity of light normally emitted, while it diminishes the light emitted in a very oblique direction. If one looks at it in an intermediate position, no appreciable effect is observed. This explains the fact, observed
in all “N” ray experiments, that only the observer placed exactly in front of the sensitive screen perceives the effect of these rays. It also shows how illusory it would be to try to make an audience witness these experiments: the effects perceived by different persons, depending as they do on their positions with regard to the screen, would certainly be contradictory or imperceptible. The rays I have called N, rays have an inverse action in all cases to that of “N” rays; they diminish the light emitted normally, and increase the light emitted tangentially. M. Macé de Lepinay (see C. R. t. cxxxviii. p. 77, January 11, 1902) has found that sound vibrations increase the glow of a phosphorescent screen as seen by an observer viewing it normally. I have noticed that if the screen is viewed tangentially, the phosphorescence is seen to decrease under the action of the sound-waves. The action of a magnetic field or of an electromotive force on a feebly luminous surface, discovered by M. C. Gutton (see C. R. t. cxxxviii. p. 268, February 1, 1904), presents the same particularities.
To sum up, in all the above-mentioned
actions, the modification undergone by the luminous emission consist in a change in its distribution along the different directions comprised between the normal and the tangent plane to the luminous surface.
On the Comparative Action of Heat and“N” Rays on Phosphorescence (March 14, 1904).
I have recently indicated that, whilst the action of “N” rays increases the quantity of light emitted by a phosphorescent screen in a normal direction, it diminishes the quantity of light emitted very obliquely (see the preceding communication). As is well known, heat also acts on phosphorescence, whose brilliancy it temporarily increases. When investigating whether this action of heat offered the same peculiarities as that of “N” rays, with regard to the direction of the emitted light, I found that, on the contrary, heat produces an increase in brilliancy in all directions comprised between the normal and the