Photography Of The Invisible. Before the Society of Arts, on Wednesday night, HIr. A. A Camp ell. Swinton reaa a paper on " 116ntgen's Photograpby of the Itvisible." Professor Dewar, -P..S., -was in the chair There -was a very large tadience. Yr. SW]XTOmq beBan by:givng an secont ef some ot the prenomena of electricl discharge in high vacua, mentioning particularly the properties of the cathode rayiJ. Passing on to deal with Rontgen's rays, he discussed the conditions On -which the opacity or trsasparency of a particular substanee seemed to dePend, ad aiso showed experimentally their property of discharging a bDdy charf,ed with eltner positive or negative electricity. Re vrent on to consider what was the best forra of Crookes-s tube to employ for. photography of the invisible, and described a new- tube from winch he had obtained stipvEior results. In it, he said, the cathode plate was eonDve, so as to focus the cathode rays upon a point on the piece of platinmn foil fornmg the anode. The foil was bent at xn angle of 40deg. to the direction of the cathode rays so as to throw dowv the RBntgen rays, which in this ce proceeded from the platinum, nor the glass. The advantges of this tibe 'were tbat it gave a sharpness of definition otherwise unattainable, and that there was less dance of the glass being cnaeked by heat. It appd to him to represent -the most inmportait advance yet made in connexion vith the apparatus required for the practical applicatioas of iRontgenas photography. As to other points in the construction of the tube usd, he said it must be made f soada, iot lead, gla6s. Further, it must be eihausted to a very hich degree, Though what exactly -was the best degree of ekhaustion was uncertain. The best plan was to, keep the tube connected with a mercury pump, so that the oehaustion could be varied at vwiL Re found that even a sealed-np tube -would not retain the same exhaustion from day to day owing to absorption of residdal gases by the electrodes, condensation on the glass, &c. In conclusion, Mr. Swia: oiv exhibited an apparatus, similar to that described by Professor r alvom, of Perugia, in which the property possessed by vioDtino-cyanide of barimn, among other sabstances, of Breing under the influence mf the, Bontgen rays was utilized so as to make it possible to see sach tbings as coms in purses, and even 'he bones inside te band. The actual process of photographing by Xdutgen's rays -was shown during the coarse o' the lecture, and a large number of photographs of the human bad and foot, fish, als, &c, were exhibited on the screen. In propobing a vote of thanks, PxDo;rssoR DEwAn said that, by his activity in bringing forward Professor Rontgen's results, Mrs Swinton aad done a real serviee to science. For of real science it was impossible to have too much populuritk, and the more the public were interested in scaentific discovery the more secare -was tho position of lingiand. At anyrate, there would be possibility of inteliectial isolation. The discovery, 3erhaps the most extraordinary of the last half-century, which 'we ow ed to Rontgen, was one to be thaiarful for bec&ase of the atimulus it had givea to scientific inquiry. 3ut it had also stimulated a hind of ert-i- cismn which he felt ought not to be entertained. This- 'was the theory that a discoverer had a monopoly in hia discovery. Iu science there was no monopoly. man -who made a discovery gave a gift to science. He himself had not tha pleasure of knowing Professor Rintgen, but-be was sure tiat the latter, so far from thinking thar Mr. Swinton in bringing forward this sub- ject was detracting from ias fame, would feel that he was but adding fresh laurels. To say thtonemanL locked within his own laboratory and, as it were, culti- vating alone his own little garden-plot, could have done as much as lhad been done by united effort in a few months was preDosterons. Euch a theory ivould retard the progress of science, and was, he thought, based on too great consideration for the personality of the dis- coverer. As to the nature of the RUntgen rays, Mr. Swinton, like other persons, was wot inclined to conmit hinself. Professor Dewar himsel felt that we -were on the dawn of new knowledge about the material medium known as the ether. ERO!POGI?AFRY OF TiB INV1SmI2.