exploring this region, by means of a narrow slit filled with phosphorescent calcium sulphide, I ascertained, without any difficulty, that, in certain azimuths, the glow of the spark diminished under the action of the rays, and increased, on the contrary, when they were intercepted by a wet screen. These were, in fact, the looked-for radiations; I will call them "N1" rays.
Although the aluminium prism of 27° 15′ I used previously is suitable for these experiments, nevertheless, in order to increase the dispersion, I used an aluminium prism of 60°, and afterwards another of 90°. With the help of the latter, I very carefully studied the feebly deviated part of the spectrum. The prism was arranged so that the angle of incidence was 20°; for each radiation, the deviation was measured and the refractive index deduced; then the wave-length was determined by means of a Brunner grating of 200 lines to the millimetre, by the process already described (see p. 57). The following table gives the numbers which result from this study, and were used for constructing the diagram (Fig. 10), in which
