The Kingdom of Man

by Sir E. Ray Lankester

The nebula (on the horizontal A A) is seen surrounded by a dark space—at the end of a long dark lane or “rift” which reminds us of the track left by a snowball rolled along in the snow. Has the nebula in some mysterious way swept up the stars in its journey through space? We cannot at present either affirm or deny such interpretations.

One or two of the brightest of the surrounding stars might just be seen by an acute eye unaided by a telescope—but no more. The best existing telescopes would show only the large nebular body on the line A A, and the larger white spots; the finest dust-like particles are stars of which the existence is only demonstrated by prolonged photographic exposures such as this, with a lens which focuses its image on to the dry plate. The old “wet-plate” would not remain wet sufficiently long to “take” the picture.

It should be borne in mind in looking at this picture that each of the minutest white spots is probably of at least the same size as our own sun: further, that each is probably surrounded by a planetary system similar to our own.

Astronomy.—A biologist may well refuse to offer any remarks on his own authority in regard to this earliest and grandest of all the sciences. I will therefore at once say that my friend the Savilian Professor of Astronomy in Oxford has turned my thoughts in the right direction in regard to this subject. There is no doubt that there has been an immense ‘revival’ in astronomy since 1881; it has developed in every direction. The invention of the ‘dry plate,’ which has made it possible to apply photography freely in all astronomical work, is the chief cause of its great expansion. Photography was applied to astronomical work before 1881, but only with difficulty and haltingly. It was the dry-plate (see [Fig. 12]) which made long exposures possible, and thus enabled astronomers to obtain regular records of faintly luminous objects such as nebulæ and star-spectra. Roughly speaking, the number of stars visible to the naked eye may be stated as eight thousand: this is raised by the use of our best telescopes to some hundred million. But the number which can be photographed is indefinite and depends on length of exposure: some thousands of millions can certainly be so recorded.

The serious practical proposal to ‘chart the sky’ by means of photography certainly dates from this side of 1881. The Paris Conference of 1887, which made an international scheme for sharing the sky among eighteen observatories (still busy with the work, and producing excellent results), originated with photographs of the comet of 1882, taken at the Cape Observatory.

Professor Pickering, of Harvard, did not join this co-operative scheme, but has gradually devised methods of charting the sky very rapidly, so that he has at Harvard records of the whole sky many times over, and when new objects are discovered he can trace their history backwards for more than a dozen years by reference to his plates. This is a wonderful new method, a mode of keeping record of present movements and changes which promises much for the future of astronomy. By the photographic method hundreds of new variable stars and other interesting objects have been discovered. New planets have been detected by the hundred. Up to 1881 two hundred and twenty were known. In 1881 only one was found; namely, Stephania, being No. 220, discovered on May 19. Now a score at least are discovered every year. Over 500 are now known. One of these—Eros—(No. 433) is particularly interesting, since it is nearer to the sun than is Mars, and gives a splendid opportunity for fixing with increased accuracy the sun’s distance from the earth. Two new satellites to Saturn and two to Jupiter have been discovered by photography (besides one to Jupiter in 1892 by the visual telescope of the Lick Observatory). One of the new satellites of Saturn goes round that planet the wrong way, thus calling for a fundamental revision of our ideas of the origin of the solar system.

The introduction of photography has made an immense difference in spectroscopic work. The spectra of the stars have been readily mapped out and classified, and now the motions in the line of sight of faint stars can be determined. This ‘motion in the line of sight,’ which was discernible but scarcely measurable with accuracy before, now provides one of the most refined methods in astronomy for ascertaining the dimensions and motions of the universe. It gives us velocities in miles per second instead of in an angular unit to be interpreted by a very imperfect knowledge of the star’s distance. The method, initiated practically by Huggins thirteen years before, was in 1881 regarded by many astronomers as a curiosity. Visual observations were begun at Greenwich in 1875, but were found to be affected by instrumental errors. The introduction of dry plates, and their application by Vogel in 1887, was the beginning of general use of the method, and line-of-sight work is now a vast department of astronomical industry. Among other by-products of the method are the ‘spectroscopic doubles,’ stars which we know to be double, and of which we can determine the period of revolution, though we cannot separate them visually by the greatest telescope.

Work on the sun has been entirely revolutionised by the use of photography. The last decade has seen the invention of the spectro-heliograph—which simply means that astronomers can now study in detail portions of the sun of which they could previously only get a bare indication.

More of the same story could be related, but enough has been said to show how full of life and progress is this most ancient and imposing of all sciences.