PHOTOGRAPHIC STAR-CHARTING.

It is now some years since photography was first called to the assistance of the astronomer, and the results which have been achieved show that it will play a still more important part in the future. A description of all its advantages would carry us far beyond the limits of the present article; but we mention four, as they are necessary to the understanding of the subject.

The power which the sensitive film possesses of recording the appearance of a bright object to whose light it has been exposed for only a minute fraction of a second, has enabled us to obtain pictures of the sun that are much more accurate than ordinary drawings. The camera, moreover, has the faculty of seeing a great deal in a very short space of time. If we confine our attention to a small area, a very few moments suffice to show us all that is to be seen by the naked eye; persistent looking for half an hour would only tire our eyes without enabling us to see anything at first invisible. It is different with the camera; the longer the light is permitted to fall on the plate, the more details do we find in the resulting picture. The fact that some rays are more effective (photographically) than others has enabled Dr Huggins to photograph, in full sunlight, that extremely faint solar appendage, the corona, which is visible to the eye only when the intense light of the sun is hidden as during a total eclipse.

The latest demand which has been made upon the astronomer’s new assistant is no less than a great atlas of all the stars down to those of the fifteenth magnitude. The magnificent idea of photographing this immense number of stars—probably about twenty millions—is due to the officials of the Paris Observatory. The instrument which Messrs Paul and Prosper Henry have constructed for this research may be described roughly as two telescopes side by side and moving together. One of these, having a specially designed object-glass, carries the sensitive plate for the reception of the image. The arrangement is provided with a clockwork motion, in order that, during the time of exposure, the situation of each star’s image may not alter; but as clockwork, however carefully made, is not infallible, an observer, looking through the second telescope, nips in the bud, so to speak, any tendency to aberration. Since the little spots that frequently occur on the photographic plates may be mistaken for stars, and so serve to swell future lists of ‘variables,’ each plate is exposed three times, and each star is therefore represented by three marks. The alteration in the position of the plate between each of the three exposures is so slight, that it requires a microscope to show that the dots are triple. With this splendid apparatus, only one two-hundredth of a second is necessary for the recording of the position of first-magnitude stars. Those of the sixth magnitude, which can only be perceived with the naked eye on a very dark night, require only half a second. The faintest which can be seen through the telescope, those of about the fourteenth magnitude, take three minutes to make an impression. But although the human eye is not sensitive enough to go any farther than this, stars of the fifteenth and even the sixteenth magnitude can be made to appear on the plate, if the exposure be sufficiently prolonged. In the latter case, an hour and a half is necessary.

In one of Messrs Henry’s charts, about five thousand stars were counted. The construction of such a chart by the ordinary method of measurement would have taken many months; now it only takes three hours. Thus the preparation of a set of maps such as Messrs Henry suggest would occupy less time than the charting of one-hundredth part of the number of stars by ordinary methods. It has been calculated that if the work be divided among twelve observatories, five hundred and ten photographs would be required from each; and making every allowance, ten years would probably see the completion of the most elaborate survey of the whole heavens ever undertaken. This may seem a long time; but we must remember that Argelander’s great charts of the northern hemisphere, which contained only three hundred and twenty-four thousand stars, occupied seven years of observatory work alone!

The importance of obtaining a permanent record of the present positions of twenty million stars cannot be overestimated. We find that if old measurements, such as those of Ptolemy, Copernicus, Tycho Brahe, and others, are to be trusted, very great changes must have taken place in the heavens. But are they to be trusted? The differences between their observations and ours must in many cases be attributed to the roughness of their instruments; but some cannot be altogether explained thus. As, however, we do not know where actual change ends and faulty measurement begins, no very definite knowledge can be derived from the comparison. A photograph of some part of the heavens by Cassini—what an invaluable legacy it would have been! With such a survey as Messrs Henry propose, future astronomers will be able not only to be sure of the existence of changes, but also to measure their extent. But the astronomers of the future will not be the only gainers. During an hour, a planet moves quite an appreciable distance; it will therefore appear on the plate as a short line instead of a point. Thus, one of the first results will probably be a considerable accession to the numbers of the minor planets which circulate between Mars and Jupiter. Who knows that the transneptunian planet itself will not be found in this way? Besides this very obvious advantage, these charts will be of the greatest use in the study of the form and constitution of the stellar universe. It is only by the employment of such charts that we can arrive at a proper understanding of star arrangement. The methods of star-gauging which Sir William Herschel employed for this purpose failed to give any satisfactory account of the form of the Milky-way in space. His first method was to point his large telescope in various directions successively, counting the number of stars visible in the field each time. He argued that if the stars were scattered with approximate uniformity throughout the galaxy, then the more he counted in a unit of area, the farther must it extend in that direction. On this assumption, he calculated that the depth of the Milky-way was eighty times the distance of the first-magnitude stars. Sir John Herschel, by precisely the same method, found the proportion to be seven hundred and fifty instead of eighty! This discrepancy alone gives some idea of the untrustworthiness of the method; and there are many other arguments against it, into which we have not space to enter.

Sir William’s second method, although sometimes confounded with his first, was in reality quite different from it. Instead of counting the number of points of light seen with the same telescope in equal small areas in various parts of the Milky-way, he now attempted to estimate the depth by noting the telescopic power necessary to ‘resolve’ the nebulous places into crowds of separate stars. When we examine the galaxy with the naked eye, it appears to be simply a cloud of misty light. A small telescope, however, suffices to show that it is made up of stars; but in most parts the background still seems nebulous. A stronger telescope entirely clears up and resolves some of these nebulous portions, while other parts require a still stronger power, and so on. In this way, then, assuming that the more difficult a misty part of the heavens was to resolve, the farther off the stars composing that misty part were, he attempted to gauge the star depths. It now appears that when he thought he was penetrating space farther and farther with telescopes of gradually increasing power, he was in reality only resolving masses of (smaller) stars situated at about the same average distance as the larger bodies which had been already distinguished with a feebler power, and which he had therefore assumed to be nearer. As a well-known writer says: ‘In each case where Herschel had assumed that he was penetrating farther and farther into space, he was only analysing more and more scrutinisingly a complex cloud of stars.’ It is interesting to notice in this connection that one still sees Herschel’s so-called split-grindstone theory (which was based on observations made by the first method) quoted and illustrated in many text-books, although he renounced it himself; and it is perfectly obvious to any one who has considered the question in the light of recent researches, that that theory is totally untenable.

The charting method gives a very different account of the constitution of the universe. Investigation in this direction has shown that the Milky-way, far from being an affair of great depth in proportion to its distance from us, is really what it seems, an immense irregular stream or belt composed of stars of all sizes. Much information has been extracted from Argelander’s great charts; but the photographic charts that will contain sixty or seventy times as many stars will be still more useful. If the idea is taken up as enthusiastically as it ought to be, and if our government, so niggardly in matters scientific, can be induced to follow the enlightened example of the Emperor of Brazil, and provide our observers with proper instruments, there is no reason why this great atlas should not be an accomplished fact in a few years.