Discoveries and Inventions of the Nineteenth Century

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 is 20,000,000,000 (twenty thousand million) times more than that of Sirius. If we suppose Sirius to be in reality only as large and as bright as our sun, it follows that its distance from us must be no less than 13,433,000,000,000 miles. The distance of stars of the 16th magnitude has been estimated to be such that their light—travelling at the rate of 185,000 miles per second—takes between five and six thousand years to reach us. For a long time no sensible parallax could be discovered in any of the fixed stars; that is, no change in their positions was discernible when viewed from points 183,000,000 miles apart, namely from the extremities of a diameter of the earth’s orbit. In other words, if we suppose the line of the length just mentioned to form the base of a triangle, having a star at its vertex, the angle formed by the sides is so small that the most refined instruments failed to measure it. In recent times, however, the parallax of a few stars—about a dozen or so—has been detected and approximately measured. The greatest observed parallax belongs to in α the constellation of the Centaur, a star of the first magnitude, 30° from the south pole of the heavens, and of this the parallax amounts to but a little more than nine-tenths of a second of angular measurement, corresponding with a distance of nearly 20,000,000,000,000 miles, a space which takes light 3½ years to pass over. This star is, therefore, believed to be the nearest of any to our system. The smallest parallax that has been measured in any of these few stars is a fraction of a second of angle corresponding with a distance twenty times greater than the other, and requiring seventy years for light to traverse it. Now, as the photographic plate shows us stars of magnitudes indefinitely smaller even than the telescopic sixteenth, we cannot but marvel at the manner in which the light travelling from these suns in the immeasurable depths of space, and taking untold thousands (nay, millions, it may be) of years in its journey is yet able so to agitate the atoms of our silver compounds that images of things that will themselves, probably, never be seen by mortal eyes are presented to our view. A circumstance requiring explanation will occur to the reader’s mind in connection with stellar photography; and that is, how does it happen that, if the image of a star is a mere point, it nevertheless impresses the plate as a visible dot? It is probably because the point is a centre whence the photographic influence radiates laterally on the plate to a small but yet sensible distance. Among the cosmic objects presented to our observation there are none more fully charged with interest and instruction than the Nebulæ. These are faintly luminous patches, in some few cases visible to the naked eye, but for the most part telescopic. The milky way, which extends round the celestial sphere, is a very conspicuous phenomenon of the same kind. A few other hazy, cloudlike patches are seen in various parts of the heavens, visible on a clear moonless night when the eye is directed towards the proper quarter. The well known group of the Pleiades sometimes presents this appearance, but most persons are able by the unassisted vision to discern in it a group of six stars at least, and an opera-glass or ordinary hand telescope easily resolves the object into a cluster of 20 or 30 distinct stars. Telescopes of higher powers bring 627more stars into view, and as many as 118 have been counted in the group. There are several other groups of this kind perceptible to the naked eyes merely as diffused patches of light, but resolvable by the telescope into thickly clustered groups of minute stars; but in many of the resolvable nebulæ the separate stars appear spread on a back-ground of diffused luminosity. Again, there are other nebulæ which telescopes of the highest powers we possess fail to resolve at all. Not only has the photographic method shown stellar components of some of these last, but it has depicted the form of nebulæ never seen at all, and whose existence was previously unknown and unsuspected. For example, the photograph has revealed the existence of a back-ground of nebulous patches to the stars of the Pleiades—a thing that had never before been suspected, although the group has been repeatedly observed by the most powerful telescopes. Those who are at all acquainted with astronomy, will understand the significance of this discovery for the science. The results already obtained afford a marvellous support to the famous speculation known as the nebular hypothesis. And as the forms of these objects are accurately shown for us by their own light, changes in their appearance may thus be detected as time goes on which may serve to lift the above named theory into the region of demonstrated truth. The nebulæ which neither telescope nor camera can resolve are such as the spectroscope proves to be masses of glowing gas or vapour. It has been already mentioned that the light from these immeasurably distant stars and nebulæ is so faint that the most sensitive photographic plates have to be exposed for hours. This would be a matter of no difficulty if the clockwork mechanism by which the apparatus is made to follow the apparent motion of the heavens could be constructed with absolute perfection. But as this is not obtainable, even with the most careful workmanship, and the smallest jar or irregularity would distort and confuse the images, this source of disturbance is eliminated in the following manner: attached to the photographing apparatus and driven with it is a telescope, provided with cross wires, and through this an observer views some star during the whole period of the exposure, his business being to keep the image of the star accurately on the cross wire, which he is enabled to do by having the means of slightly modifying the movement of the clockwork. In the Paris Exhibition of 1889 were shown many very fine large photographic prints of nebulæ (notably of great nebula in Orion), which have recently been obtained in this manner, and those nebulæ that had been photographically resolved had the stellar components marked with wonderful distinctness. Comets and meteorites have been photographed, and even the aurora borealis and the lightning’s path have been brought within the camera’s ken. Space would fail us to describe the many applications now found for photography in microscopy, in medicine and surgery, in anthropology, in commerce, and in the arts. It is obvious also from the improvements that are continually made, that many of these applications have not yet received their full developments. Photography has been enlisted into the service of the army and navy, and regular courses of instruction in the art are given in their training schools. A well equipped photographic waggon now accompanies every army corps, and in almost every ship of war, some proficient operator is to be found. By an ingenious combination of photography, aerostatics and electricity, it is possible to obtain with perfect safety accurate information of the disposition of an 628enemy’s forces and fortifications. A small captive balloon is sent up, to which is attached a camera. At a height of a few hundred yards, the balloon is practically safe from any projectiles, and in its cable are interwoven two electric wires by which currents are conveyed to electro-magnets, which produce all the movements required for any number of exposures. Jurisprudence has found its account in recognizing the art, for the photograph is received in evidence for proving identity, etc. The administration of the criminal law takes advantage of the art to secure the likeness of prisoners for future identification, and the modern instantaneous process renders unnecessary the subjects’ concurrence with the operation. Again, if the “hue and cry” has to be raised for an individual “wanted” for any offence, and a photographic likeness of him is procurable, thousands of copies can be made of it in a few hours, by night as easily as by day, and distributed to every police station in the whole country. Modern processes now enable us to obtain prints from negatives in as many seconds as a few years ago hours were required, and this by artificial light. A process of printing lately introduced and yielding artistic results which deserve to find more general favour, is that called the platinotype. Instead of the ordinary print produced on lightly glazed paper by the reduction of silver compounds, and of questionable permanency, the image is formed in the paper by metallic platinum, the most changeless of all possible substances under ordinary influences. The pictures are of a rich velvety black, with soft gradations, and the surface is without glaze or glare. The print has, in fact, the appearance and all the best qualities of the most highly finished mezzotint engraving, combined with the minute fidelity characteristic of the photograph. The problem of producing a photograph in colours, permanently showing nature’s tints in all their gradations, has still a great fascination for some experimenters, and startling announcements are made from time to time of some discovery in this direction. It does not appear, however, that any success has