state of molecular strain is transitory, as is also the emission of “N” rays. Torsion produces effects analogous to compression. er On the Dispersion of “N” Rays and on their Wave-length (January 18, 1904). To study the dispersion and the wavelengths of “N” rays, I used methods quite similar to those employed for light. In order to avoid complications which might have resulted from the storing-up of “N” rays, I used exclusively prisms and lenses of aluminium, a substance which does not absorb their rays. The following is the method employed to study dispersion. The rays are produced by a Nernst lamp, enclosed in a lantern of sheet-iron, pierced with an opening, which is shut by aluminium foil ; the rays from the lamp which pass through this opening are sifted by a deal board 2 cms. thick, a second sheet of aluminium, and two leaves of black paper, so as to eliminate radiations foreign to “N” rays. In front of those screens, and at a distance of 14 cms. from the lamp filament, a large screen of wet cardboard is arranged, in which a slit has been cut 5 mms. wide and 3.5 cms. high, exactly opposite the lamp filament. In this way I obtain a welldefined pencil of “N” rays; this pencil is received on an aluminium prism whose refractive angle is 27° 15', placed so that one of its faces is normal to the incident pencil. It is, then, possible to prove that from the other refractive face of the prism several pencils of “N” rays, horizontally dispersed, emerge. For this purpose a slit i mm. broad and i cm. high, cut in a sheet of cardboard, is filled with calcium sulphide rendered phosphorescent ; by displacing this slit, the position of the dispersed pencils is determined without difficulty, and the deviations being known, their refractive indices are deduced. This is the method of Descartes. I thus established the existence of “N” radiations, whose indices are respectively 1 '04, l'19, 1'29, 1'36, 1:40, 1:48, 1:68, 1.85. In order to measure with more exactness the first two indices, I made use of another aluminium prism having an angle of 60°. I again found for one of the indices the same value, 1'04; and for the other, l'15 instead of 1'19. In order to control the results obtained by the prisms, I determined the indices by producing, by means of an aluminium lens, images of the lamp filament, and measuring their distances from the lens. The lens, which is planoconvex, has a radius of curvature of 6.63 cms., and an aperture of 6.8 cms. The slit of the wet screen is widened so as to form a circular opening 6 cms. in diameter; the lens is placed at a known distance (cm.) from the incandescent filament, and by means of the phosphorescent sulphide, the position of the conjugate images of the filament is determined. The following table gives the values of the indices found, both with the prism and the lens :Prisms. Lens. 29° 15' - 600 þ= 40 p= 30 p= 22 1.85 1.86 I'91 1991 1.68 1.67 1.66 1.67 1:48 1:40 1'42 1:43 1'36 1'36 1936 1937 1'29 ... 1936 1931 I'19 1050 I'44 1:48 1.42 I'20 I'04 Here is another verification of these results : if for the fourth index the mean value 1:42 is adopted, one works out that for an aluminium prism of 60°, the incidence giving the minimum deviation is 45° 19', and that this deviation is 30° 38'; the observed deviation was 31° 10'. With the same incidence, the calculated deviation of the radiation, whose index is 1.67, is 57° 42'; the observed deviation was 56° 30'. I now pass on to the determination of wave-lengths. By means of the above-described arrangement for studying dispersion by the prism of 27° 15', refracted pencils are obtained, each of which is sensibly homogeneous. If we make the pencil we wish to study impinge on a second screen of wet cardboard, pierced with a slit 1'5 mm. wide, we can isolate a narrow portion of this pencil. On the other hand, a piece of aluminium foil is fixed to the moving radial arm of a goniometer, so that its plane is normal to the arm ; in this foil a slit is cut only 0.07 mm. wide, and filled with phosphorescent calcium sulphide ; the goniometer is arranged so that its arn axis is exactly underneath the slit of the second wet cardboard. By turning the arm, the path of the pencil is exactly marked out, and one can verify that it is quite unique, and is accompanied by no lateral pencil, such as diffraction could eventually produce in the case of large wave-lengths. A grating is then placed in front of the slit of the second wet cardboard (for instance, a Brunner grating of 200 lines per mm.). If, now, the emerging pencil is explored by turning the arm which bears the phosphorescent sulphide, the existence of a system of diffraction fringes is confirmed, just as with light, only these fringes are much closer together, and are sensibly equidistant. This already indicates that “N” rays have much shorter wave-lengths than luminous radiations. The angular distance of the fringes, or what amounts to the same thing, the rotation of the arm corresponding to the passage of the phosphorescent slit from one luminous fringe to the next, is very small. It is therefore determined by the method of reflection, with the aid of a divided scale and telescope, a way |