EVOLUTION OF THE TELESCOPE.
The development of telescopic power within the present century is one of the most striking examples of intellectual progress and mastery in the history of mankind. The first day of the century found us, not, indeed, where we were left by Galileo and Copernicus in the knowledge of the skies and in our ability to penetrate their depths, but it did find us advanced by only moderate stages from the sky-lore of the past.
The after half of the eighteenth century presents a history of astronomical investigation and deduction which confirmed and amplified the preceding knowledge; but that period did not greatly widen the field of observation. If the sphere of space which had been explored on the first day of January, 1801, could be compared with that which is now known and explored by our astronomers, the one sphere would be to the other even as an apple to the earth.
It is difficult to apprehend the tremendous strides which we have made in the production of telescopes and the consequent increase in our sweep of the heavens. It was only in 1774 that the elder Herschel began his work in the construction of reflecting telescopes. These he gradually increased in size, until near the close of the century, when he produced an instrument which magnified two hundred and twenty-seven diameters. In the course of his career he built two hundred telescopes, having a seven-foot focus; 150 of ten feet and about eighty of twenty feet each.
With these instruments the astronomical work in the last quarter of the eighteenth century was mostly performed. The study of the heavens at this epoch began to reach out from the planetary system to the fixed stars. In this work Herschel led the way. The planet Uranus at first bore the name of Herschel, from its discoverer. Sir John Herschel, son of Sir William, was born in 1792. All of his astronomical work was accomplished in our century. Following the line of his father, he used the reflecting telescope, and it was an instrument of this kind that he took to his observatory at the Cape of Good Hope. Lord Rosse was born in the year 1800. Under his auspices the reflecting telescope reached its maximum of power and usefulness. His great reflector, built in his own grounds at Birr Castle, Ireland, was finished in 1844. This instrument was the marvel of that epoch. It had a focal distance of fifty-three feet, and an aperture of six feet. With this great telescope its master reached out into the region of the nebulæ, and began the real work of exploring the sidereal heavens.
In the reflecting telescope, however, there are necessary limitations. Before the middle of this century, it was known that the future of astronomy depended upon the refracting lens, and not on the speculum. The latter, in the hands of the two Herschels and Rosse, had reached its utmost limits—as is shown by the fact that to this day the Rosse telescope is the largest of its kind in the world.
Meanwhile the production of refracting telescopes made but slow progress. As late as 1836 the largest instrument of this kind in the world was the eleven-inch telescope of the observatory at Munich. The next in importance was a nine and a half-inch instrument at Dorpat, in Russia. This was the telescope through which the astronomer Struve made his earlier studies and discoveries. His field of observation was for the most part the fixed and double stars. At this time the largest instrument in the United States was the five-inch refractor of Yale College. Soon afterward, namely, in 1840, the observatory at Philadelphia was supplied with a six-inch refracting telescope from Munich.
German makers were now in the lead, and it was not long until a Munich instrument having a lens of eleven inches diameter was imported for the Mitchell Observatory on Mount Adams, overlooking Cincinnati. About the same time a similar instrument of nine and a half inches aperture was imported for the National Observatory at Washington. To this period also belongs the construction of the Cambridge Observatory, with its fifteen-inch refracting telescope. Another of the same size was produced for the Royal Observatory at Pulkova, Russia. This was in 1839; and that instrument and the telescope at Cambridge were then the largest of their kind in the world.
The history of the telescope-making in America properly begins with Alvan Clark, Sr., of Cambridgeport, Massachusetts. It was in 1846 that he produced his first telescope. Of this he made the lens, and such was the excellence of his work that he soon became famous, to the degree that the importation of foreign telescopes virtually ceased in the United States. Nor was it long until foreign orders began to arrive for the refracting lenses of Alvan Clark & Sons. The fame of this firm went out through all the world, and by the beginning of the last quarter of the century the Clark instruments were regarded as the finest ever produced.
We cannot here refer to more than a few of the principal products of Clark & Sons. Gradually they extended the width of their lenses, gaining with each increase of diameter a rapidly increasing power of penetration. At last they produced for the Royal Observatory of Pulkova a twenty-seven-inch objective, which was, down to the early eighties, the master work of its kind in the world. It was in the grinding and polishing of their lenses that the Clarks surpassed all men. In the production of the glass castings for the lenses, the French have remained the masters. At the glass foundry of Mantois, of Paris, the finest and largest discs ever produced in the world are cast. But after the castings are made they are sent to America, to be made into those wonderful objectives which constitute the glory of the apparatus upon which the New Astronomy relies for its achievements.
It was in the year 1887 that the Lick Observatory on Mount Hamilton, of the Coast Range in Southern California, was completed. The lens of this instrument is thirty-six inches in diameter. Nor will the reader without reflection readily realize the enormous stride which was made in telescopy when the makers advanced from the twenty-seven-inch to the thirty-six-inch objective. Lenses are to each other in their power of collecting light and penetrating apace as the squares of their diameters, and in the extent of space explored as the cubes of their diameters.
The objective of the Pulkova instrument is to that of the Lick Observatory as 3 is to 4. The squares are as 9 is to 16, and the cubes are as 27 is to 64. This signifies that the depth of space penetrated by the Lick instrument is to that of its predecessor as 16 is to 9, and that the astronomical sphere resolved by the former is to the sphere resolved by the latter as 64 is to 27—that is, the Lick instrument at one bound revealed a universe more than twice as great as all that was known before! The human mind at this one bound found opportunity to explore and to know a sidereal sphere more than twice as extensive as had ever been previously penetrated by the gaze of man.
Nor is this all. The ambition of American astronomers and American philanthropists has not been content with even the prodigious achievement of the Lick telescope. In recent years an observatory has been projected in connection with the University of Chicago, which has come almost to completion, and which will bear by far the largest telescopic instrument in the world. The site selected for the observatory is seventy-five miles from the city, on the northern shore of Lake Geneva. There is a high ground here, rising sufficiently into a clear atmosphere, nearly two hundred feet above the level of the lake.
The observatory and the great telescope which constitutes its central fact are to bear the name of the donor, Mr. Yerkes, of Chicago, who has contributed the means for rearing this magnificent adjunct of the University. The enterprise contemplated from the first the construction of the most powerful telescope ever known. The manufacture of the objective, upon which everything depends, was assigned to Mr. Alvan G. Clark, of Cambridgeport, Massachusetts, who is the only living representative of the old firm of Alvan Clark & Sons.
Alvan G. Clark has inherited much of the genius of his father, though it is said that in making the lens of the Lick Observatory the father had to be called from his retirement to superintend personally some of the more delicate parts of the finishing before which task his sons had quailed. But the younger Clark readily agreed to make the Geneva lens, under the order of Yerkes, and to produce a perfect objective forty inches in diameter! This important work, so critical—almost impossible—has been successfully accomplished.
The making and the mounting of the Yerkes telescope have been assigned to Warner & Swasey, of Cleveland, Ohio, who are recognized as the best telescope builders in America. The great observatory is approaching completion. The instrument itself has been finished, examined, accepted by a committee of experts, and declared to fulfill all of the conditions of the agreement between the founder and the makers. Thus, just north of the boundary line between Illinois and Wisconsin, the greatest telescope of the world has been lifted to its dome and pointed to the heavens.
The formal opening of the observatory is promised for the summer months of 1896. The human mind by this agency has made another stride into the depths of infinite space. Another universe is presently to be penetrated and revealed. A hollow sphere of space outside of the sphere already known is to be added to the already unthinkable universe which we inhabit. Every part of the immense observatory and of the telescope is of American production, with the single important exception of the cast glass disc from which the two principal lenses, the one double convex and the other plano-concave, are produced. These were cast by Mantois, of Paris, whose superiority to the American manufacturers of optical glass is recognized.
It is estimated that the Yerkes telescope will gather three times as much light as the twenty-three-inch instrument of the Princeton Observatory. It surpasses in the same respect the twenty-six-inch telescope at the National Observatory in the ratio of two and three-eighths to one. It is in the same particular one and four-fifth times as powerful as the instrument of the Royal Russian Observatory at Pulkova; and it surpasses the great Lick instrument by twenty-three per cent.
What the practical results of the study of the skies through this monster instrument will be none may predict. Theoretically it is capable of bringing the moon to an apparent distance of sixty miles. Under favorable circumstances the observer will be able to note the characteristics of the lunar landscape with more distinctness than a good natural eye can discern the outlines and character of the summit of Pike's Peak from Denver. The instrument has sufficient power to reveal on the lunar disc any object five hundred feet square. Such a thing as a village or even a great single building would be plainly discernible.
Professor C.A. Young has recently pointed out the fact that the Yerkes telescope, if it meets expectation, will show on the moon's surface with much distinctness any such object as the Capitol at Washington. It is complained that in America wealth is selfish and self-centred; that the millionaire cares only for himself and the increase of his already exorbitant estate. The ambition of such men as Lick of San Jose and Yerkes of Chicago, seems to ameliorate the severe judgment of mankind respecting the holders of the wealth of the world, and even to transform them from their popular character of enemies and misers into philanthropists and benefactors.