"N" Rays/On the Dispersion of "N" Rays and on Their Wave-Length

by Prosper-René Blondlot

On the Dispersion of "N" Rays and on their Wave-length (January 18, 1904).

To study the dispersion and the wave-lengths 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 well-defined 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 1 mm. broad and 1 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, 1.19, 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, 1.15 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 plano-convex, 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 ($p$ 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. ⁠ ⁠ ⁠ 27° 15′ 60° ⁠ $p=40$ $p=30$ $p=22$ 1.85 {{{1}}}„ ... ... 1.86 1.91 1.91 1.68 {{{1}}}„ ... ... 1.67 1.66 1.67 1.48 {{{1}}}„ ... ... 1.50 1.44 1.48 1.40 {{{1}}}„ ... ... 1.42 1.42 1.43 1.36 {{{1}}}„ ... ... 1.36 1.36 1.37 1.29 {{{1}}}„ ... ... 1.36 1.31 1.37„ 1.19 1.15 ... ... 1.20 1.31„ 1.37„ 1.04 1.04 ... ... 1.20„ 1.31„ 1.37„

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 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 plane-mirror being fixed to the arm. Moreover, one measures, not the distance between two consecutive fringes, but that between two symmetrical fringes of a high order—for example, that between the tenth fringe on the right, and the tenth fringe on the left. From these measures of angle, and from the number of lines per millimetre of the grating, the wave-length can be deduced by the known formula.

Each wave-length has been determined by three series of measures, effected with three gratings, having respectively 200, 100, and 50 lines per millimetre.

The following table exhibits the results of these measures:—

		Wave-lengths.
		$\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$
		Grating employed.
		$\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$						Probable values deduced from the preceding.
Indices.		200 lines per mm.		100 lines per mm.		50 lines per mm.
1	·04	0	·00813	0	·00795	0	·00839	0	·00815
1	·19	0	·0093	0	·0102	0	·0106	0	·0099
1	·4	0	·0117		—		—	0	·0117
1	·68	0	·0146		—		—	0	·0146
1	·85	0	·0176	0	·0171	0	·0184	0	·0176

Being desirous of controlling these determinations by the use of a quite different method, I had recourse to Newton's rings. These being produced, in yellow light, for instance, if one passes from one dark ring to the following, the variation of optical retardation in air is one wave-length of yellow light. If, now, with the same apparatus and the same incidence, rings are produced by means of "N" rays, and the number of these rings comprised between two dark rings in yellow light is counted, we shall obtain the number of times which the wave-length of "N" rays is contained in the wave-length of yellow light. This method, applied to rays of index 1.04, gave the values 0.0085 instead of 0.0081 found by the gratings; and for the index 1.85, the value 0.017 instead of 0.0176. Though the ring method is inferior to the grating method, on account of the uncertainty attending the exact position of the dark rings in the experiment, an uncertainty which is due to the necessity of rendering these rings very wide, the concordance of the numbers obtained by the two methods constitutes a valuable control.

In the tables given above I have retained all the decimals occurring in the calculation of the numbers deduced from observation. Although I cannot with certainty indicate the degree of approximation of the results, I believe, nevertheless, that the relative errors do not exceed 4 per cent.

The wave-lengths of "N" rays are much smaller than those of light. This is contrary to what I had imagined for a moment, and contrary to the determinations which M. Sagnac thought he had deduced from the position of the multiple images of a source, obtained with a quartz lens, images attributed by him to diffraction. I had previously observed that while polished mica lets "N" rays pass, roughened mica stops them, and also that whereas polished glass reflects them regularly, ground glass diffuses them. These facts were already an indication that "N" rays could not have large wave-lengths. If we desire to study the transparency of a body, we must take care that the surface is well polished. Thus I had at first classed rock-salt amongst opaque substances, because the specimen I used, having been sawn from a large block, had remained unpolished; in reality, rock-salt is transparent. 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.