seem to be destined to replace the less facile visual methods which were employed exclusively until a few years ago. The heliometer (q.v.) is generally known as the most exact instru- ment for executing measurements on the sky. Yet, in the opinion of some of the highest au- thorities, the equality of photographic results with those of the heliometer may now be con- sidered as conclusively demonstrated. The year 1892 saw the first complete publication of an extensive series of results. These were derived from excellent photographs of the cluster Plei- ades, made in 1872 and 1873 by Rutherfurd, in New York. These results have been followed in the last few years by many other star-cluster measures, so that an extensive mass of material is being gathered by photography — material of the highest precision and of the last importance in stellar astronomy.
It is only by a study of minute inter-stellar changes in "the star-clusters that we may hope, within the limits of human time, to throw some light upon the problems of motion within the greater universe that lies beyond our own solar system: and it is to photography that wo must look for our observational material. Generaliza- tions of science can be secured only by the discussion of very large masses of observations; but although the heliometer can give us suffi- cient precision, it involves so much labor that by its exclusive use astronomy could not hope to do more than touch the surface of those great problems.
An account of the immense advances made, since the introduction of photography, in our knowledge of planetoids is given in a special article concerning these bodies. Suffice it to mention here that the photographic revelations in this important field have culminated in the discovery of the planet Eros ( q.v. ) , now knowTi to be, with the exception of the moon, our near- est neighbor in space. Jlany comets, too, have been discovei'ed, and many doubtless will be discovered with the aid of photography: and it is photography again that furnishes the means of determining their positions and meas- uring their orbits with greater facility than could be obtained by any other method at pres- ent known. There is no doubt, further, that photography will greatly increase the list of known distances between the earth and remote heavenly bodies (see Parallax), and that be- fore many years it will replace the eye even in noting the instant of a star's transit across the meridian of an observatory. See Transit Instrument.
But decidedly the most complete triumph of astro-photography is Gill and Kapte^'s Cape of Good Hope photographic Durchmusterung, the results of which are comprised in a remark' ably complete catalogue of the southern heavens. In the year 1882, a very bright comet appeared in (he sky, and was especially conspicuous in the Southern Hemisphere. Its brightness led Gill, at the ('a|)e of Good Hope Observatory, to attempt to photograph it. Having no special photographic apparatus, he called to his aid a neighboring portrait photographer named Allis. An ordi- nary i)ortrait camera was employed by them, and, for convenience in mounting, it was simply strapped to the tube of a visual telescope in the observatory. With this ap])aratus they ob- tained what is believed to be, in all probability, the earliest successful photograph of a comet. In examining the negative, Gill's attention was attracted to the large number of stars appear- ing on the plate as minute points or dots, and it was these that first suggested to him the possibility of making a complete examination of the heavens with the aid of photography. The German term Durchmusterunfi, now natu- ralized in scientific English, was first applied to Argelander's (q.v.) great catalogue of stars in the northern half of the sky, a work which has rendered possible many statistical and other researches of far-reaching importance in stellar astronomy. But Argelander and his successor, SchiJnfeld, observing at Bonn, in Germany, were able to carry on their survey only a short way beyond the northern half of the sky. Gill, in his obseiTatory at the Cape of Good Hope, far south of the equator, saw the possibility of extending .4rgelander's work, by means of pho- tography, to the South Pole. To have so ex- tended it by the visxial methods that were used iy Argelander would have required an enor- mous expenditure of time and labor: but a small photographic telescope was procured, and, in the hands of C. I?ay Woods, a scientific pho- tographer who was summoned from England, a complete collection of plates of the southern sky was made. The negatives were sent to Groningen, Holland, and were there measured by Kaptera. The resulting catalogue, pub- lished in three large volumes by the British Government, is found to possess even greater completeness than Argelander's; for the fallible human eye could not avoid omitting a star now and then, w-hile the photographic plate is of course subject to no such error.
The Cape Durchmusterung is, however, not the only great result to which Ciill's 1882 comet photograph has led. The Durchmusterung was carried out much on the same lines as Arge- lander's older work in the Xorthern Hemisphere, the idea being to make a census of all the stars in the sky down to a given magnitude, even if it should be impossible to give more than a rough approximation of their positions in the sky. But the success of the first enterprise led Gill to conceive the idea of preparing a new star catalogue that should satisfy the condition of high precision as well as that of complete- ness as to numbers. So vast an undertaking coiild be rendered possible only by the com- bined efforts of many astronomers and many observations, and, as the result of preliminary correspondence, a meeting to consider the sub- ject was called at Paris by the French Govern- ment in 1887. Delegates from all the civilized nations attended, and it was decided that the proposed photographic catalogue of precision should really be made, that it might stand for all time as the foundation of research in sid- ereal astronomy. Eighteen observatories have since been engaged in carrying out this astro- nomical enterprise, the greatest the world has ever seen. To make error of any kind practi- cally impossible, the photographs are taken in duplicate, the total number of plates required being 44,108, each representing a surface of four square degrees. All stars down to the eleventh magnitude, numbering about 2,000,000, will thus be subjected to precise measurement. A fur- ther series of plates is being made for the myriads of stars of still lower brilliancy. But
