Part 9

With the then existing methods of computation of orbits it was imperative to have numerous measured positions to use as data for the calculation. The scanty data available in the case of Ceres were not sufficient for the application of the method. The occasion discovered a man, one of the greatest mathematicians of the nineteenth century, Karl Frederick Gauss, who, although only twenty-five years of age, undertook the solution of the problem by employing a system which he had devised, known as “the method of least squares,” which enabled him to obtain a most probable result from a given set of observations.

This, with a more general method of orbit computation, also elaborated by himself, was sufficient to enable him to calculate future positions of Ceres, and on the anniversary of the original discovery, Olbers, another great pioneer in orbit calculations, found the planet in very nearly the position assigned by Gauss. So great was the curiosity regarding the other portions of the planet, which was supposed to have been shattered, that numerous observers at once commenced to search after other fragments.

These were the _actualities_ of 1801 and thereabouts; but the seed of much future work was sown. Kant and Laplace had already occupied themselves with theories as to the world formation, and spectrum analysis as applied to the heavenly bodies may be said to have been started by Wollaston’s observations of dark lines in the solar spectrum in 1802. Fraunhofer was then a boy at school. In the same year the first photographic prints were produced by Wedgewood and Davy.

OBSERVATORIES

It has been stated that at the beginning of the century there were no permanent observatories either in the southern hemisphere or in the United States. The end of the century finds us with two hundred observatories all told, of which fourteen are south of the equator and forty-seven in the United States, among which latter are the best-equipped and most active in the world.

The observatory of Parramatta was the first established (1821) in the southern hemisphere. This was followed by that at the Cape of Good Hope in 1829. Of the more modern southern observatories from which the best work has come we may mention Cordova, the seat of Gould’s important investigations, established in 1868, and Arequipa, a dependency of Harvard, whence the spectra of the southern stars have been secured, erected still more recently (1881).

I believe, but I do not know, that the large number of American observatories have radiated from Cincinnati, where, in consequence of eloquent appeals, both by voice and pen, from Mitchell, then professor of astronomy, an observatory was commenced in 1845. There can be no doubt that at the present moment, with the numerous well-equipped and active observatories, and the careful and thorough teaching established side by side with them, which enables numberless students to use the various instruments, the United States, in matters astronomical, fills the position occupied by Germany at the beginning of the century.

In Europe special observatories have been established at Meudon, Kensington, and Potsdam, so that new astrophysical inquiries may be undertaken without interfering with the prosecution or extension of the important meridional work carried on at Paris, Greenwich, and Berlin. A large proportion of the observations made by the Lick and Yerkes observatories in the United States has been astrophysical.

One of the special inquiries committed to the charge of the Solar Physics Observatory at Kensington at its establishment by the British government had relation to the possibility of running home meteorological changes on the earth, especially those followed by drought and famines in various parts of the empire, to the varying changes in the sun indicated by the ebb and flow of spots on its surface. With this end in view observations of the sun were commenced in India and the Mauritius to supplement those taken at Greenwich. At the same time other daily observations of sun spots by a different method were commenced at Kensington.

This kind of work was at first considered ideally useless; we shall see later on what has become of it.

IMPROVEMENTS IN TELESCOPES

The progress in astronomical science throughout the nineteenth century has naturally to a great extent depended upon the advances made both in the optics of the telescope and the way in which they are mounted, either with circles to record exact times and positions, or made to move so as to keep a star or other celestial objects in the field of view while under observation. The perfection of definition and the magnitude of the lenses employed in the modern instrument have been responsible for many important discoveries.

Ever since the telescope was invented—Galileo’s lens was smaller than those used in spectacles—men’s minds have been concentrated on producing instruments of larger and larger size to fathom the cosmos to its innermost depths.

At the beginning of the century we were, as we have seen already, in possession of reflectors of large dimensions; Herschel’s four-foot mirror, the instrument he was using in 1801, which had a focal length of forty feet, was capable of being employed with high magnifying powers; and it was the judicious use of these, on occasions when the finest of weather prevailed, that enabled him to enrich so extensively our knowledge of the stellar and planetary systems. For the ordinary work of astronomy, however, especially when circles are used, refractors are the more suitable instruments. This form suffers less from the vicissitudes of weather and temperature, and is, therefore, more suited where exact measurements are required.

Towards the end of the eighteenth century a Swiss artisan, Pierre Guinard, after many years of patient labor, succeeded in producing pure disks of flint glass as large as six inches in diameter. The modern refracting telescope thus became possible.

In 1804 there was started at Munich the famous optical and mechanical institute, which soon made its presence felt in the astronomical world. Reforms in instrument making were soon taken in hand, and under the leadership of the great German astronomer, Bessel, great strides were made in instruments of precision. Fraunhofer, who had been silently working away at the theory of lenses, and making various experiments in the manufacture of glass, was joined, in 1805, by Guinard. In 1809 Troughton invented a new method of graduating circles, according to Airy the greatest improvement ever achieved in the art of instrument making.

In 1824 Fraunhofer successfully completed and perfected an object-glass of 9.9 inches in diameter for the Dorpat Observatory. This objective might literally have been called a “giant,” for nothing approaching it in size had been previously made.

England, which was at one time the exclusive seat of the manufacture of refracting telescopes, was now completely outstripped by both Germany and France, and for this we had to thank “the short-sighted policy of the government, which had placed an exorbitant duty on the manufacture of flint glass.” In 1833 the Dorpat refractor was eclipsed by one of fifteen inches aperture made for the Pulkowa Observatory by Merz & Mähler, Fraunhofer’s successors, who about ten years later supplied a similar instrument to Harvard College. At that time Lord Rosse emulated with success the efforts of Herschel and rehabilitated the reflector by producing a metallic mirror of six-foot aperture and fifty-four-foot focal length which he mounted at Parsonstown. The speculum weighed no less than four tons. To mount this immense mass efficiently and safely was a work of no light nature, but he successfully accomplished it, and eventually both mirror and the telescope, which weighed now altogether fourteen tons, were so well counterpoised that they could be easily moved in a limited direction by means of a windlass worked by two men. The perfection of the “seeing” qualities of this instrument and its enormous light-grasping powers were particularly striking, and observational astronomy was considerably enriched by the discoveries made with it.

Speculum metal was not destined to stay; ten years later (1857) the genius of Léon Foucault introduced glass mirrors with a thin coating of silver deposited chemically, and these have now universally superseded the metallic ones.

The long supremacy of Germany in the matter of refractors was broken down ultimately by the famous English optician and engineer, Thomas Cooke, of York. His first considerable instrument, one of seven inches aperture, was finished in 1851; and in 1865, a year before his lamented death, he completed the first of our present giant refractors, one of twenty-five inches aperture, for Mr. Newall, of Gateshead. In consequence of the success of Cooke’s achievement other large refractors were soon undertaken.

Alvan Clarke, the famous optician of Cambridgeport, Massachusetts, at once commenced a twenty-six-inch for the Washington Observatory. The next was one of twenty-seven inches, made by Grubb for the Vienna Observatory. Object-glasses now grew inch by inch in size, depending on the increased dimensions of disks that could be satisfactorily cast. Gautier, of Paris, completed a twenty-nine-and-a-half-inch for the Nice Observatory, while Alvan Clarke made an objective of thirty inches for Pulkowa. In 1877 the latter successfully completed the mounting of an objective of thirty-six inches for the Lick Observatory, but this immense lens was only achieved after a great number of failures. Even this large object-glass was surpassed in size by the completion in 1892 of the forty-inch which he made for the Yerkes Observatory, and by that made by Gautier for the Paris Exhibition of 1900.

So much, then, for the largest refractors. In recent years, since the introduction of the silver on glass mirrors, with their stability of figure and brilliant surface, which can be easily renewed, reflectors of large apertures are again being produced. The first of these was one of thirty-six inches aperture made by Calver for Dr. Common, who demonstrated its fine qualities and his own skill by the beautiful photographs of the nebula of Orion he was enabled to secure with it. Dr. Common himself has since turned his attention to the making and silvering of large mirrors of this kind, and the largest he has actually completed and mounted equatorially is one with a diameter of five feet. Another of thirty-six inches aperture is in use at the Solar Physics Observatory at Kensington.

The progress of depositing silver on glass has led of late years to important developments in which _plane_ mirrors are used. Foucault was the first to utilize such mirrors in his “siderostat,” in which such a mirror is made to move in front of a horizontal fixed telescope, which may be of any focal length, and no expensive dome or rising floor is required. The plane mirror of the siderostat in the Paris Exhibition telescope is six feet in diameter.

A variation of this instrument is the cœlostat more recently advocated by Lippmann. The Coudé equatorial mounting also depends upon the use of plane mirrors; with such a telescope the observer is at rest at a fixed eye-piece or camera in a room which may be kept at any temperature.

Now that in astronomical work eye observations are indispensably supplemented by the employment of photography, an important modification of the refracting telescope has become necessary; this was first suggested by Rutherfurd.

The ordinary achromatic object-glass consists, as a rule, of two lenses, one made of flint and the other of crown glass; but in this form the photographic rays are not brought to the same focus as the visual rays. This, however, can be achieved by employing three lenses instead of two, each of different kinds of glass. The most modern improvement in the telescope is due to Mr. Dennis Taylor, of Cooke & Sons, and to Dr. Schott and Professor Abbe, whose researches in the manufacture of old and new varieties of optical glass have rendered Mr. Taylor’s results feasible. By the Taylor lens outstanding color is abolished, all the rays being brought absolutely to the same focus; such lenses can therefore be used either for visual observations or for photography for spectroscopy.

SPECTROSCOPIC ASTRONOMY

The branch of physics which at the present day has assumed such mighty and far-reaching proportions in astronomical work is that dealing with spectrum analysis, which, although suggested as early as the time of Kepler, did not receive any impetus as regards its application to celestial bodies until the beginning of the present century at the hands of Wollaston and Fraunhofer. Then, however, it still lacked the chemical touch supplied afterwards by Kirchhoff and Bunsen. They showed us that the spectrum observed when the light from any heated body is passed through a prism is an index to the chemical composition of the light source; the constitution of a vapor when in a condition to absorb light can be determined by an extension of the same principle, first demonstrated by Stokes, Angström, and Balfour Stewart, when the century was about half completed.

The first celestial body towards which the spectroscope was turned was our central luminary, the sun.

Wollaston first discovered that its spectrum was crossed by a few dark lines; we learned next from Fraunhofer, who in 1814 worked with instruments of greater power, that the solar spectrum was crossed not only by a few dark lines, but by some hundreds. Not content with examining the light of the sun, Fraunhofer turned his instrument towards the stars, the light of which he also examined, so that he may be justly called the inventor of stellar spectrum analysis. It is not to the credit of modern science that from this time forward spectrum analysis did not become a recognized branch of scientific inquiry, but, as a matter of fact, Fraunhofer’s observations were buried in oblivion for nearly half a century. The importance of them was not recognized till the origin of the dark lines, both in sun and stars, had been explained by Stokes and others, as before stated. The lines in the solar spectrum were mapped with great diligence by Kirchhoff in 1861 and 1862, and later by Angström and Thalen, and this was done side by side with chemical work in the laboratory. The chemistry of the sun was thus to a great extent revealed; it was no longer a habitable globe, but one with its visible boundary at a fierce heat, surrounded by an atmosphere of metallic vapors, chief among them iron, also in a state of incandescence. To these metallic vapors Angström added hydrogen shortly afterwards.

Here, then, was established a firm link between the heavens and the earth; the first step to the problem of the chemistry of space had been taken.

It was only natural that as advances were made the instrumental equipment should keep pace with them. Spectroscopes were built on a larger scale; more prisms, which meant greater dispersion, were employed to render the measurements of the lines in spectra more accurate. The growth of our knowledge especially necessitated the making of maps of the lines in the solar spectrum, and in the spectra of the chemical elements which had been compared with it on a natural scale. This was done by Angström, who utilized for this purpose the diffraction grating invented by Fraunhofer, and defined the position of all lines in spectra by their “wave lengths,” in ten-millionths of a millimetre or “tenth-metres.”

In 1862 Rutherfurd extended Fraunhofer’s work on the stars by a first attempt at classification. Two years later Huggins and Miller produced maps of the spectra of some stars. Donati demonstrated that comets gave radiation spectra, and Huggins did the same for nebulæ.

By these observations comets and nebulæ were shown to be spectroscopically different from stars, which at that time were studied by their dark lines only.

Chiefly by the labors of Pickering, the energetic head of the Harvard Observatory, science has been enriched during the later years by observations of thousands of stellar spectra, the study of which has brought about the most marvellous advance in our knowledge.

These priceless data have enabled us now to classify the stars not only by their brightness, or their color, but by their chemistry.

Next to be chronicled is the application of the so-called Doppler-Fizeau principle, which teaches us that when a light source is approaching or receding from us the light waves are crushed together or drawn out, so that the wave length is changed. The amount of change gives us the velocity of approach or recess, so that the rate of movement of stars towards or from the earth, or the up-rush or down-rush of the solar vapors on the sun’s disk can be accurately determined. A further utilization of this principle is found when the stars are so close together that they appear as one if the plane of motion passes near the earth. A line common to the spectra of both stars will appear double twice in each revolution, when the motion to or from the earth, or, as it is termed, “in the line of sight,” is greatest. “Spectroscopic doubles,” as these stars are called, yield up many of their secrets which otherwise would elude us. Their time of revolution, the size of the orbit, and the combined mass can be determined.

To return from the stars to the sun.

By the device of throwing an image of the sun on the slit of the spectroscope the spectra of solar spots have been studied from 1866 onward, and a little later the brighter portions of the sun’s outer envelopes, revealed till then only during eclipses, were brought within our ken spectroscopically, so that they are now studied every day.

CELESTIAL PHOTOGRAPHY

Wedgewood and Davy, in 1802, made prints on paper by means of silver salts, but it was not until 1830 that Niepce and Daguerre founded photography, which Arago, in an address to the French Chamber, at once suggested might subsequently be used to record the positions of stars.

In 1839 we find Sir John Herschel carrying out a series of experiments so important for our correct knowledge of the sequence of steps in the early stages of photography that I have no hesitation in quoting from one of Herschel’s manuscripts relating to a deposit on a glass plate of “muriate” [chloride] of silver from a mixed solution of the nitrate with common salt. The manuscript states: “After forty-eight hours [the chloride] had formed a film firm enough to bear draining the water off very slowly by a siphon. Having dried it, I found that it was very little affected by light, and by washing it with nitrate of silver, weak, and drying it, it became highly sensitive. In this state I took a camera picture of the telescope on it.”

The original of the above-mentioned photograph, the first photograph ever taken on glass, is now in the science collection at the Victoria and Albert Museum, South Kensington.

In the early days of photography colored glasses were first used to investigate the action of different colors on the photographic plate. Sir John Herschel was among the first to propose that such investigations should be made direct with a spectrum, and he, like Dr. J. W. Draper, stated that he had found a new kind of light beyond the blue end of the spectrum, as the photographic plate showed a portion of the spectrum there which was not visible to the eye. Advance followed advance, and in 1842 Becquerel photographed the whole solar spectrum, in colors, with nearly all the lines registered by the hand and eye of Fraunhofer, not only the blue end, but the complete spectrum, from Draper’s “latent light,” as he called the ultra-violet rays, to the extreme red end.

The first photograph of a celestial object was one of the moon, secured by Dr. J. W. Draper in 1840; we had to wait till 1845, so far as I know, before a daguerreotype was taken of the sun; this was done by Foucault and Fizeau, while the first photograph of a star—Vega—was taken at Harvard in 1850. After the introduction of the wet-collodion process regular photography of the sun’s surface was commenced, at Sir John Herschel’s recommendation, at Kew in 1858, and the total solar eclipse of 1860 was made memorable by the photographs of De La Rue, who before that time had secured most admirable photographs of the moon, as also had Rutherfurd.

Photography now began to pay the debt she owed to spectrum analysis.

The first laboratory photograph of the spectra of the chemical elements was taken by Dr. W. A. Miller in 1862.

Rutherfurd was the first to secure a photograph of the solar spectrum with considerable dispersion by means of prisms.

In 1863 Mascart undertook a complete photographic investigation of the ultra-violet portion of the solar spectrum, a work of no mean magnitude. He, however, did not employ a train of prisms for producing the spectrum, but a diffraction grating, using the light reflected from the first surface. The first photograph of the spectrum of a star was secured by Henry Draper, the son of Dr. J. W. Draper, one of the pioneers in photography in 1872.

It was not till the introduction of dry plates in 1876 that the photography of the fainter celestial objects or of their spectra was possible, as a long exposure was naturally required. Stellar spectra were photographed by Huggins in 1879, and in the next year Draper photographed the nebula of Orion. As the dry plates became more rapid, and as longer exposures were employed, revelation followed revelation; the nebulæ as seen by the naked eye, and even some stars, were found by the Henrys, Roberts, Max Wolf, Barnard, and others, to be but the brighter kernels of large nebulous patches.

This new application of photography, depending upon long exposures (the longest one I know of has extended to forty hours), had an important reflex action on the mechanical parts of the telescope; it was not only necessary to keep the faintest star exactly on the same part of the plate during the whole of the exposure, but night after night the stellar image must be brought on to the same part of the plate so that the exposure might be continued.

A system of electric control of the going of the driving-clock of the telescope by means of a sidereal clock was introduced, the simplest one being designed by Russell, of Sydney; a most elaborate one by Grubb, of Dublin.

Another application of the method of long exposures has been the discovery of minor planets by the trails impressed by their motion among the stars on the photographic plates on which the images of both are impressed.

A complete spectroscopic survey of the stars by means of photography was commenced in 1886 at Harvard College, as a memorial to Draper, who died while he was laboring diligently and successfully in securing advances in astrophysical inquiries. To carry on this work at Harvard, Professor Pickering wisely reverted to the method first employed by Fraunhofer, and utilized by Respighi and another in 1871, of placing prisms in front of the object-glass.

In the photographing of stellar spectra by means of objective prisms, the driving-clock of the telescope must _not_ go exactly at sidereal rate, but at certain speeds depending on the brightness and position of the star under examination.