Discoveries and Inventions of the Nineteenth Century

from its appearance—namely gun-cotton and camphor! This material is prepared in a plastic condition that enables it to be shaped into any required form. It can be drawn into threads or rolled out into very thin films. Thin plates of it have been used in photography as a substitute for glass, for the sake of lightness, before its employment as a transparent film in the roll-holders. We have now at length the equipment of the travelling photographer reduced to the utmost conceivable limits of lightness and compactness. Thus the complete apparatus 623required for taking hundreds of pictures of a good size need not be more than a few pounds in weight, and can easily be carried in the hand. But even quite small negatives can now be very readily printed in a few seconds on paper, with an enlargement of many times the original dimensions. The resources of the photographic art appear indeed to be endless; but a mere statement of even the more interesting of these would lead us beyond our limits, and descriptions of the details of manipulation are out of our province altogether. But a few of the more recent applications and developments of the art scarcely or not at all alluded to in the foregoing pages should receive some attention. The extraordinary sensitiveness of the gelatine-bromide film which makes it possible to impress on it a photographic image in the merest fraction of a second of time, enables us to take pictures of objects in rapid motion. Express trains at their highest speed have been successfully photographed, and so has almost every moving object in nature. The photographs that have been taken of men, of birds, horses, and other animals in every phase of their most rapid actions, have solved many disputed and perplexing problems as to the nature of their movements, and sometimes the solutions have been of a very unexpected kind. Taking a photographic “shot” at a bird has become almost more than a figure of speech; for there are contrivances by which a bird on the wing may be aimed at with the lens, and hit off on the sensitive plate with a certainty surpassing that of the fowling-piece. There are also photographic repeaters by which six or more successive photographs of the bird, etc., can be taken in a single second. Mr. Muybridge has published a number of such photographs of the horse, and by projection of the different images on a screen from a magic lantern, in rapid succession, he has been able to reproduce the visual appearance of horses trotting, leaping, galloping, etc., on the principle of the zoetrope (page 399). Photography has afforded wonderfully delicate observations in many departments of science, by recording phenomena too rapid for the eye to seize, or too recondite for direct perception. A few examples may be mentioned. First, the advantage of photographing the lines of spectra, such as those described in our article on the spectroscope, will at once suggest themselves, and accordingly this method of recording spectra has been largely used, and in the hands of Mr. Lockyer, Dr. Draper, and others has been successfully applied to the study of the solar and stellar spectra. But more than this, it is the sensitive photographic plate that has enabled us to explore the region of the solar spectrum lying far beyond its visible limits in the red and in the violet rays. The ultra-violet portion of the spectrum is shown photographically to be occupied by multitudes of the thin insensitive spaces—breaks in the continuity of the active rays—which are impressed on the photographic print as black lines, similar in every respect to the lines mapped out in the visible spectrum by Fraunhofer. It is known by these that the ultra-violet spectrum, produced by glass prisms, extends to a distance beyond the last visible rays of nearly double the space occupied by the colour spectrum. The principal lines, or rather the greater groups of lines in the invisible spectrum, are distinguished by the capital letters of the alphabet, in continuation of Fraunhofer’s method, beginning from H and nearly exhausting the letters of the alphabet to designate them. These are photographed in the dark; for all the solar beams that are allowed to enter the stereoscope are first passed through blue glass of such a depth 624that every kind of emanation capable of affecting the human eye is intercepted. Another extremely interesting example of the application of the art to scientific research is celestial photography. An image of the sun may be impressed on a sensitive plate in an ordinary camera, in an amazingly short space of time, but such image is much too small to show any of the markings on the disc of our luminary, even when the image is magnified, for its diameter is only about ⅒th of an inch for each 12 inches of the focal length of the lens. In order to obtain an image of 4 inches diameter, a lens of 40 feet focal length must therefore be used. The first attempts in solar photography appear to have been made in France, in 1845, and the solar prominences were daguerrotyped in 1851; but it was not until 1860, that Mr. De La Rue succeeded in obtaining some beautiful negatives of the phenomena presented in an eclipse of the sun, and was thus enabled to determine a great astronomical problem, by showing that the red flames, or prominences, really belonged to the sun itself. At the present time, photographs of parts of the sun’s disc are regularly taken at Kew, and other observatories, without the very long and heavy telescopes, which introduced many mechanical difficulties into the operation; for, by means of Foucault’s siderostat, the great lens and the photographic apparatus can be used in one fixed position. The siderostat is an instrument on which a flat mirror, made of glass worked to a perfect plane and silvered externally, is caused by clockwork to follow the motion of the sun, so that the reflected beams can be projected in any required direction unchangeably, and, therefore the image of the sun (or other heavenly bodies) viewed in the mirror, is absolutely stationary. The lens, carried in a short tube, has its axis directed to this image, just as it would be pointed at the luminary itself. In solar photography, the exposure is made through a very narrow slit in an opaque screen, which is caused to move rapidly in front of the image. Very fair photographic images of the sun, of several inches diameter, can, however, be obtained with an ordinary telescope of five feet or so focal length, by substituting a small photographic lens and camera in the eye-piece, and by enlarging the image in printing. As early as 1840, Dr. Draper succeeded in daguerrotyping the moon, but it was not until 1851, that lunar photographs, obtained by Professor Bond, another American astronomer, were first exhibited in England. Many other distinguished experimenters have since successfully turned their attention to this subject, such as Dancer, of Manchester, Secchi, Crookes, Huggins, Phillips, and De La Rue. The latter, and also Mr. Fry, by photographing the moon, at different periods of her libration, have obtained very beautiful and interesting stereoscopic prints of our satellite, in which she presents to the eye the roundness and solidity of a cannon ball. Mr. Rutherford, in America, had an object glass of 11¼ inches diameter, made expressly with correction for the chemical rays, and with this instrument he has produced some of the finest photographs of the moon that have yet been taken. Reflecting telescopes, which have the advantage of uniting all the rays in one focus, have been used with excellent results, and it is said that some taken with the great reflector at Melbourne, where also the atmospheric conditions are very favourable, are almost perfect. Excellent photographs of the planets have also been taken by Mr. Common and others; but they are of course small, and have contributed 625so far, much less to our astronomical knowledge than those already mentioned. Very different are the results obtained in what, a short time ago, appeared a less promising field. The image of a so-called fixed star, in even the most powerful telescopes, presents itself as a mere luminous point, and this is the case whether the star is one of the brightest or one of the least conspicuous. The telescopic appearance is simply a more or less brilliant point. The various degrees of brightness which distinguish one star from another (stella enim a stellâ differt in claritate), and which the unassisted eye attributes to difference of size, led, long before the invention of telescopes, to a classification of them accordingly. The brightest stars are said to be of the 1st “magnitude,” those of the next inferior degree of brilliancy, of the 2nd “magnitude,” and so on, down to the 6th, which includes the faintest star discernible by an acute eye under favourable circumstances. But stars too faint to be thus seen came into view in the field of the telescope, and therefore those of the 7th magnitude, and beyond, are termed telescopic stars, and each additional power given to the instrument brings others in view that previously were invisible. The classification has been carried down to the 18th or 20th magnitude, which expresses the limit of visibility with the most powerful telescopes yet constructed. In the methods hitherto employed for this classification, there is necessarily much that is arbitrary and vague, and it is quite common to find a different magnitude assigned to the same star by different authorities. Now the photographic plate enables the astronomer to determine the relative brightness of stars quite definitely. Everyone knows that the time required to impress an image on the sensitive plate is longer, as that image is less luminous. Hence, by finding the time required for the images of different stars to be impressed, we have a measure of their relative luminosities. Suppose the image of a group of stars is allowed to act on a plate for, say, 5 seconds, we should find only the brightest stars represented. If a second plate have double the exposure given, it would be impressed by the images of not only the brightest stars of the group, but also by those of the next degree of brilliancy; and a third plate exposed for 20 seconds would show more stars than the two former exposures. So that plate after plate might be exposed under the same group for successively longer and longer intervals indefinitely. Exposures extending over hours have been made, notably by Mr. Common in England, and by Mr. Gill at the Cape of Good Hope, showing not only how magnitude may be determined to any extent, and the heavens most accurately mapped out, but with this very remarkable result:—thousands of stars, invisible even in the most powerful telescopes, are portrayed in the photographs. Let us consider for a moment the significance of this fact with regard to the new space-exploring powers it has placed in the hands of science. The number of stars visible to the unassisted eye in the whole expanse of the heavens has been variously estimated, but the figures usually given lie between 3,000 and 4,000, and the highest estimate for the most acute eyesight, under the most favourable atmospheric conditions, places the limit at 5,000. The brightest star in the heavens is Sirius, and Sir. J. Herschel ascertained that its light is about 324, that of an average star of the 6th magnitude. Taking the average luminosities of stars of the first six magnitudes, Sir W. Herschel, from his own observations, represents their relative brightness by the following figures: 100; 25; 12; 6; 2; 1. The different degrees of brightness seen is, probably, due to the following 626three causes, combined in various proportions: (1) the different sizes of these luminaries themselves; (2) differences in their intrinsic luminosity; and, (3) differences in their distances from us. And it is also extremely probable that the last is generally by far the largest factor of the three. It has been found by photometrical experiments, that the light we receive from the sun