Discoveries and Inventions of the Nineteenth Century

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 gems, and “in gloss of satin and glimmer of pearls.” The eye is also the organ which conveys to our minds the impressions of visible motion, with all those pleasures of exciting spectacle which enter so largely into our enjoyment of life. It likewise discriminates the forms, sizes, and distances of objects; but by a process long misunderstood, and dependent upon a set of perceptions which, although precisely those whence we derive our most fundamental notions of the objects around us, have been completely overlooked in that time-honoured enumeration of the senses which recognizes only five. If such be the extent to which our minds are dependent upon the wonderful apparatus of the eye, it may easily be imagined what must be the comparative narrowness of mental development in those who have never enjoyed this precious sense, and the feeling of deprivation in those, who, having enjoyed it, have unfortunately lost it. Well may our sublime poet despairingly ask— “Since light so necessary is to life, And almost life itself—if it be true That light is in the soul— The all in every part: why was the sight To such a tender ball as the eye confined, So obvious and so easy to be quenched?” —for he himself, in his own person, experienced this deprivation, and he thus touchingly, in his great work, laments his loss: “Thus with the year Seasons return; but not to me returns Day, or the sweet approach of even or morn, Or sight of vernal bloom, or summer’s rose, Or flocks, or herds, or human face divine; But cloud instead, and ever-during dark Surround me; from the cheerful ways of men Cut off; and for the book of knowledge fair Presented with a universal blank Of Nature’s works—to me expunged and rased, And wisdom at one entrance quite shut out.” An organ which is the instrument of so many nice discriminations as is the eye must, of course, present the most delicate adjustment in its parts. So much has in recent times been learnt of the nature of its mechanism; of the relation between the impressions made upon it and the judgments formed by the mind therefrom; of the illusions which its very structure produces; of the defects to which it is liable; and of its wonderfully 454refined physiological elements—that a branch of science sufficiently extensive to require a large part of a studious lifetime for its complete mastery has grown up under the hands of modern physiologists, physicists, and psychologists. To some of the results of their labour we would invite the reader’s attention; and in order to render the account of them intelligible, we must, to a certain extent, describe “things new and old.” THE EYE. Fig. 234.—Vertical Section of the Eye. The form of the human eye and the general arrangement of its parts may be understood by referring to Fig. 234, which is a section of the eyeball. It has a form nearly globular, and is covered on the outside by a tough firm case, A, named the sclerotic coat, which is, for the most part, white and opaque. This covering it is which forms what is commonly termed the “white of the eye;” but in the front part of the eyeball it loses its 455opacity, and merges into a transparent substance, termed the cornea, B. The cornea has a greater convexity than the rest of the exterior of the eyeball, so that it causes the front part of the eye to have a somewhat greater projection than would result from its general globular form. This sclerotic coat—with its continuation, the cornea—serves to support and protect the more delicate parts within, and is itself kept in shape by the humours, which fill the whole of the interior. The greater space is occupied by the vitreous humour, C; but the space immediately behind the transparent cornea is filled with the aqueous humour, D. The latter is little else than pure water, and the former is like thin transparent jelly. The cavities containing these two humours are separated by the transparent double convex lens, E, called the crystalline lens, which, in consistence, resembles very thick jelly or soft gristle. The outward surface of this lens has a flatter curvature than the inner surface. Immediately in front of the crystalline lens is found the iris, F, which may be described as a curtain having in the middle a round hole. The iris is the part which varies in colour from one individual to another—being blue, brown, grey, &c.; and the aperture in its centre is the dark circular spot termed the pupil. The general disposition of the parts of the eye with regard to light will be most easily understood by comparing it with an optical instrument, to which it bears no little resemblance, namely, the camera obscura, so well known in connection with photography. We may picture to ourselves a still more complete resemblance, by imagining that the lens of the camera is single, that we have fixed in front of it a watch-glass, with the convex side outwards, and that we have filled with water the whole of the interior of the camera, including the space between the watch-glass and the lens. The focussing-screen of the camera corresponds with the inner surface of the back of the eyeball, about which we shall presently have more to say. Now, even if the camera had no lens, but were simply a box filled with water, and having in front the watch-glass, fixed in the manner just mentioned, we could obtain the images of objects on the screen, as a consequence of the curvature of the watch-glass. It would, however, in this case, be necessary to have the camera much longer, or, in other words, the rays would be brought to a focus at a greater distance than if we put in the glass lens, which would, thus placed in the water, cause the rays to converge to a focus at a much shorter distance, although its effect when surrounded by water would be less powerful than in the air. There we see the effect of the crystalline lens of the eye in bringing the rays to a focus within a much shorter distance than that which would be required had there been present only the curved cornea, and the aqueous and vitreous humours of the eye, which are but little different from pure water in their optical properties. If we focus the camera by adjusting the distance between the lens and the screen so as to get a distinct image of a near object, we should find, on directing the instrument to a distant one, that the image would be blurred and indistinct, and the lens would have to be moved nearer to the screen; or we could get the image of the distant object distinct by replacing the lens by another