Christmas Lectures For The Young. The theatrd of the Royal Institution was crowded to its full capacity on Saturday afternoon, when 1rofessor Silvanus P. Thompson delivered the sixth and last of his Chridsmas lectures on " Visible and Invisible Light." The subject was Rontgen Light. Pno:Fssont Tnompsoce said that so nany fabulous stories had been printed about the circumstances in which Rontgen had made his discovery tnat it might be well to give Rontgen's own account of the matter. It was on OctoberSi, 1895, that henoticed somethingnew. Re was experimentingwithaCrook-es's tube enveloped in arn opaque black covering, when he observed that a piece of paper painted with barium platino-cyanide was being affected. On investigation he was obliged to assume that the Crookes's tube was responsible for the phenomenon, snd,in fact,found that from the tubethere was proceeding some influence which could pass through black paper and affect a luminescent screen even at a distance of a couple of yards. There was not, there- fore, as had been widely stated, a line of photography in the matter, still less any newr photography. What was new was a new kind of light which, untike the -inds already known, could neither be reflected nor refracted nor polarized. Rbntgen folowed up his dis- covery by taking the fairly obvious atep of trying the effect of this new light on old-fashioned photographic dry plates, but even here there was no " new photo- grapoy " m i proper sense. Later on he found that many substances opaque to ordinary light were transparent tu this kind, and observed that the flesh was trans- I parent tu it-more so than the bones. The lecturer then took lP.ntgen Photographs of the hands of two juvenile members of the audience, and showed how the different transparency of true and false gems to Rhintgen light could be used as a means of distinguish- ing those which were sham. 7o discovery, he con- tinued, stood alone, and Rltntgen's Vas no exception to the general rule. Ilis rays were obtained from a Crookies's tube, and Croolces's tubes were not the first vazuum tubes. Two centuries ago a great sensation was caused by the discovery thal light -as produced on sha!;ing a tube containing mercury and exhausted of air. Professor Thompson proceeded to exhibit the various effects of light produceet in vacuum tubes by the electric discharge, and shoved how they altered in character according to the degree of exhaustion, the best effects requiring high exhaustion. A magnificent Geissler tube was exhibited, and Sprengel's mercurial pump with Crookes's improvements exulained. Reference was also made to the work of Hitiorfl, who notieed that in a highly-rarefied tube the discharge would not go round a corner. Some of the plenomena of Crookes's tubes and cathode light were next broaght forward, and various luminescent and mechanical effects exhibited. Ihe power of the cathode rays to cast a shadow was ullustrated, together with a curious Phenomenon described a week or two ago by Professor Flemmg, who bas noticed that if ra Crookes's tube, constrected to show the cathode shadow, be exposed to the influence of a magnerxc field, the shadow becomes distorted and smaller, the dim outlino of another shadow becoming at the same t:me visible in another part of the tube. Pass ng to Leuard's e rleSments with Crookes's tubes, tho lecturer described his attempts to obtain cathode rays outside the tube in air, and told how, follow.rng an observation made by Hertz, he found thev would penetrate a window of aluminium foil let into tie glass of the tube. Plintgen began where Lenard left off, and observed the rays now named after him. One other property of these rays was mentioned-their power of iselectlifying electrified bodies. Ihis they aodxib do whether th- body was charged positively or negatively, therein differing from ultra-vioLet light, which possessed the sam_e powver in the caso of negative electrification only. Some of the latest improved forms of tuibes for producing ltontgen rays wrere described and exhib:ted, and reference made to the so-called inventions of the cryptoscope and the flnorescope, based on the facts observed and described by Podntgen himself. In con- clusion, a large number of photographs of the bones of the hands and feet, of the skeletons of various animals and even of anew-born baby, and of articles enclosed in bhxes, &c., wvere exhibited in the lantern, and by means of the flnoresceut screen opporturity was given of I seeing the invisible." CHRJSTMAS LBCTURBES FOR THB YOUNG.