Discoveries and Inventions of the Nineteenth Century

apparatus, as they are completely permeable to the X rays, and their whole contents may be rendered useless. The impression obtained on the photographic plate is not so much a photograph as a shadow of the interposed object—a shadow more or less dense in the positive print according to the permeability of the object, and the length of the exposure. These photographic results have sometimes been called “shadowgrams,” “radiograms,” “radiographs,” “skiagraphs,” etc. The word skiagraph appears the most appropriate designation. That the emanations from the phosphorescing substance on which the cathode rays impinge are entitled to be also called rays, appears from the regularity of the shadows thrown on the fluorescent screen or photographic plate; and the fact of their propagation in straight lines was proved by Dr. Roentgen obtaining a pin-hole photograph of the phosphorescing part of the vacuum tube, when the latter was enveloped 448in black paper. Why a pin-hole and not a lens was used for taking this photograph will presently appear. Fig. 232b.—Skiagraph of a Hand, by Dr. Roentgen. The Third Finger has a Ring on it. One of Dr. Roentgen’s experiments excited the attention and interest of the general public, as well as of the scientific world, in the most extraordinary degree, and though its announcement was received in some quarters with incredulity, experimenters in all parts of the world immediately set themselves at work to test the truth of the alleged discovery. Electrical apparatus of different kinds, with various adjustments, were employed, with results that were in some cases failures, in others confirmations of the German professor’s statements, and not unfrequently the variations in the conditions gave rise to increased knowledge of the phenomena generally. The experiment just alluded was one in which a dry photographic plate contained in one of the camera dark slides, now so familiar to every one, was placed (with the slide still closed by its wooden cover of nearly one quarter of an inch thick) a few inches below the Crookes’ tube, and the hand of a living-person being extended on the outside of the cover, a shadow of the bones of the hand, as if seen through the surrounding tissues, was obtained. Much popular misconception as to the powers of the “new photography” arose from want of knowledge of the process by which these strange pictures were obtained, the common notion being that these photographs were produced by some method of using a camera, and that outlines of people’s bodies and skeletons could be taken instantaneously, not only through their clothes, but through doors and walls. Much nearer the mark was the allusion of a scientific writer as to the possibility of the new process realising Dickens’ description 449of Marley’s ghost: “His body was transparent, so that Scrooge, observing him, and looking through his waistcoat, could see the two buttons on his coat behind.” The value of the new discovery for medical and surgical purposes was immediately recognised, and very soon its application was successfully practised. Dr. Roentgen found that the X rays are incapable of refraction, in this respect differing from ordinary light (see pages 397 and following), and among the experiments which most impressed and astonished his auditors when he was lecturing at Potsdam on his new discovery before the Imperial Court of Germany, was one in which he showed the X rays passing in a straight line through water without undergoing refraction. The rays pass without interruption equally through the substances, whether these be coherent or in a layer of fine powder of the same thickness, and this again shows that there can be no regular reflection or refraction. Prisms and lenses, whether of glass, ebonite, or aluminium, fail to afford evidence of refractive action, hence the X rays cannot be focused like those of ordinary light, and that is why the photograph of the vacuum tube had to be taken by a pin-hole. Again, glass lenses could not be used, because this substance, so transparent to light, is particularly opaque to the X rays, and would in a great degree intercept them, while lenses of ebonite and of aluminium, which were tried, were inoperative on account of the irrefrangibility of the rays. As to the nature of the rays themselves, Dr. Roentgen rejects the notion of their being “ultra-violet” rays, which was suggested by some. The meaning of this term is seen when it is understood that a great distance beyond the violet end of the visible spectrum there are radiations, revealed by their photographic impressions, so that the whole spectrum is really some eight times as long as the visible part. In consequence of these ultra-violet rays acting on the photographic plate, it is possible, as has long been known, to take a photograph in the dark. The eye is quite insensible to the X rays also, and although these, as we have seen, readily pass through the bodily tissues, it may be placed quite near the discharge tube, the latter being enveloped in black paper, without causing any sensation. That the new rays are in some way allied to light is the opinion held by Dr. Roentgen, and he is inclined to consider them as due to longitudinal vibrations in the ether; that is, instead of the transverse waves to which light is attributed, these resemble the waves of sound, in so far that they move in the direction of propagation. This would account for the absence of any distinct refraction, or polarisation, which seems to characterise the X rays. Their connection with certain electric Maxwell-Hertz waves (see p. 541) is more problematical, as the mathematical formulæ for these admit only transverse oscillation. But on the assumption of certain conditions, due to the action of electricity, etc., on highly rarefied air, the possible existence of longitudinal vibrations has been deduced by admitting a certain variation in some of the factors of the Maxwell formulæ. Other suggestions have been advanced in order to make the observed facts concerning the X rays fit into established theories, but so far these attempts have been unsuccessful. It would seem as if our present conceptions of light, electricity, the ether and matter, will have to be profoundly modified and enlarged in order to bring these and other recently discovered phenomena within their scope. Since the publication of Dr. Roentgen’s paper, his results have received confirmation in every quarter, 450and many new observations have been added, some of which seem to tend not so much to elucidate the phenomena, as to prove them even more complicated than was at first supposed. Such was the announcement in June, 1896, of the discovery of several varieties of X rays. Fig. 232c.—Metal Objects taken through Calico and Sheet of Aluminium. In the meantime, various modifications have been made in the forms of the tubes and electrodes, and divers arrangements have been used for the exciting electrical apparatus. Thus it has been found that the X rays are given off from platinum more copiously than from glass, aluminium, or any other substance, and by using a tube closed by a “platinum window,” on which the cathode rays impinge, Mr. Gifford has been able to reduce the time of exposure for obtaining a skiagraph of the bones on the hand to half a minute, whereas twenty times that period was formerly required. Another form of tube is advertised by Brady & Martin of Newcastle, with which, in conjunction with a new screen, a coil giving a 5–inch spark will, it is stated, yield a good skiagraph of the hand in two seconds, which appears to be the shortest time yet attained. Another firm of tube-makers, Newton & Co., London, state that their special form of tube, excited by a coil giving a 6–inch spark, and used with their fluorescent screen, “will work right through the human body, showing the heart, liver, spine, ribs, the movements of the heart and of the 451diaphragm, etc.” It has recently been observed that the best results are obtained when there is a certain, but as yet undefined, relation between the degree of rarefaction of the residual gas in the tube, and the intensity or frequency of the electric discharges, and that these should be accommodated to the work required. Thus, for example, if a skiagraph of the hand be attempted with an apparatus in which these factors are carried to too high a degree, the resulting X rays will pass through the bones almost as freely as through the surrounding tissues, and their shadows will therefore not appear. If, on the other hand, the contrary conditions hold, an incomplete or maybe no result will be found. This seems to explain the failures that have sometimes occurred when tubes of apparently identical construction have been used in the hands of the same, or of different, observers. Perhaps more depends also on the time of exposure. For instance, if a short exposure be given in the case of the hand, the photograph will be merely a silhouette of that member; with a little longer exposure, this will show the nails; with still longer time, the shadow of the fleshy parts begins to grow faint and the skeleton to appear. With yet more prolonged exposure only the bones will show, in their various degrees of opacity, and the shadows of these will gradually disappear as the time of exposure is increased, until at length the image will be entirely effaced. The considerable differences as to distinctness of the various tissues, which are exhibited by the published prints of hand shadows, are thus explicable. Fig. 232d.—A Skiagraph of Layers of Various Substances. 452 Fig. 233.—Portrait of Professor Helmholtz. SIGHT. The investigations of modern science have borne rich fruit, not only by vastly extending our knowledge of the universe of things around us, but also making us acquainted with the mode in which certain agents act upon our bodily organs, and by revealing, up to a certain point, what may be termed the mechanism of that most wonderful thing—the human mind—or, at least, that part which is immediately concerned in the perceptions of an external world. Of all the physical influences which affect the human mind, those due to light are the most powerful and the most agreeable. One of the most ancient of philosophers says, in the simple words which are appropriate to the expression of an undeniable truth, “Truly the light is sweet, and a pleasant thing it is for the eyes to behold the sun.” The impression produced by light alone is a source of pleasure—a cheering influence of the highest order; and there is a special character in the pleasing effects of light, from the circumstance that they do not exhaust the sense so quickly as do even pleasurable impressions on other organs—such as sweet tastes, fragrant odours, or agreeable sounds. Sight is not liable to that satiety which soon overtakes the enjoyment of sensations arising 453from the other senses; it possesses, therefore, a refinement of quality of which the rest are devoid. Sight converses with its objects at a greater distance than does any other sense, and it furnishes our minds with a greater variety of ideas. Indeed, our mental imagery is most largely made up of reminiscences of visual impressions; for when the idea of anything is brought up in our minds by a word, for example, there arises, in most cases, a more or less vivid presentation of some visible appearance. Our visual impressions are also longer retained in memory or idea than any other class of sensations. The nature of the impressions we receive through the eye is extremely varied; for we thus perceive not only the difference between light and darkness, but in the sensations of colour we have quite another class of effects, while the lustre and sparkle of polished and brilliant objects add new elements of beauty and variety. We find examples of the latter qualities in the verdant sheen of the smooth leaf, in the splendid reflections of burnished gold, in the bright radiance of glittering