“When the glass walls, instead of receiving cathode rays, are pierced only by the true Roentgen rays from the disk in the middle, no evidence is afforded by my photograph that the glass under these circumstances acts as a source. It is well that it does not, for its only effect would be a blurring one. § 91. With focus tubes, the glass phosphoresces under the action of the X-rays as anything else would phosphoresce, but its phosphorescence is not of the least use. It is a sign that a tube is working well, and that the rays are powerful; but if by reason of fatigue (§ 58) the glass ceases to phosphoresce strongly, the fact constitutes not the slightest detriment.” [Illustration] X-RAY UNINFLUENCED BY A MAGNET. SEVERE TEST.—His first experiment on magnetic deflection, the sciagraph of a magnet with a background of wire gauze, only showed that if there were any shift by reason of passage of rays between the poles it was very small; but he definitely asserted, as in accompanying diagram, that a further experiment has been made which effectually removes the idea of deflectibility from his mind, and confirms the statement of Professor Roentgen. § 79. A strong though small electro-magnet, with concentrated field, had a photograph of its pole-pieces taken with a couple of wires, A and C, stretched across them on the further side from the plate—nearer the source—and a third wire, B, also stretched across them, but on the side close to the plate. These three wires left shadows on the plate, of which B was sharp and definite, while A and C were blurred. Two sciagraphs were taken by Mr. Robinson, one with the magnet on, and one with the magnet reversed. On subsequently superposing the two plates, with the sharp shadows of B coincident, the very slightest displacement of shadows A and C could have been observed, although those shadows were not sharp. But there was absolutely no perceptible displacement, the fit was perfect. Consequently the hypothesis of a stream of electrified particles is definitely disproved as no doubt had already been effectively done in reality by Professor Roentgen himself. But it must be noted, he stated, that the hypothesis of a simple molecular stream—not an electrified one—remains a possibility. The only question is whether such an unelectrified bombardment would be able to produce the observed effects. It must be remembered, Dr. Lodge stated, that Dr. Lenard found among his rays two classes as regards deflectibility—some much deflected, others less deflected; and it must be clearly understood that his deflections were observed, not in the originating discharge tube, where the fact of deflection is a commonplace, but outside, after the rays had been, as it were, “filtered” through an aluminum window. He did not, indeed, observe the deflection in air of ordinary density; it was in moderately rarefied air that he observed it, § 72_a_, but he showed that the variation of