THE TELESCOPE.
Since much that we know about the heavenly bodies, has been revealed to us through the medium of the telescope, it may be advisable to give some slight account of this instrument. As early as 1608 it appears to have been invented in Holland by a professor of mathematics named James Metius, though the honor is claimed by their friends for several other parties, among them Lipperhey and Jansen, spectacle makers in the town of Middleburg. The claims of Jansen were supported by Peter Borelli, in a small volume published in 1655, entitled “De Vero Telescopii Inventore,” and he was for a long time regarded as the inventor. The story runs that Jansen had shown a telescope sixteen inches long to Prince Maurice and Archduke Albert, who realizing the importance of such an invention in war, induced him to keep it a secret. But the narrative given by Borelli rested on such a slight foundation, that it obtained but little credence. Later evidence shows that Hans Lipperhey, on the 2d of November, 1608, made application to the states-general of Holland for a patent for “an instrument to see with at a distance,” but was refused on the ground that the invention was already known. While there is little doubt but that the discovery was, as claimed, made in Holland, it is also highly probable that great efforts were put forth for some time to keep the matter a profound secret. At least, no results were published to the world until made known by Galileo in the manner thus related by Professor Newcomb: “About six months after the petitions (for patents to the states-general of Holland) of Lipperhey and Metius, Galileo was in Venice on a visit, and there received a letter from Paris, in which the invention was mentioned. He at once set himself to the re-invention of the instrument, and was so successful that in a few days he exhibited to the astonished authorities of the city a telescope magnifying three times. Returning to his home in Florence, he made other and larger ones which revealed to him spots on the sun, the phases of Venus, the mountains of the moon, the satellites of Jupiter, the seeming handles of Saturn, and some of the myriads of stars, separately invisible to the naked eye, but whose combined light forms the milky way. But the largest of these instruments magnified only about thirty times, and was so imperfect in construction as to be far from showing as much as can be seen with a modern telescope of the same power.”
The telescope has been aptly compared to an eye. In the eye nearly parallel rays of light fall on a lens, and this lens throws an image. In the telescope, nearly parallel rays of light fall on a lens, and this lens throws an image, and then another lens enables the eye to form an image of that image by again rendering the rays parallel; these parallel rays entering the eye just as rays do in ordinary vision. The efficiency of the telescope depends on its power of illuminating and magnifying. If the object glass (in a “refractor,” as an ordinary spy-glass, the lens next the object viewed) be twenty times greater than the pupil of the eye, it receives twenty times more light, and forms an image theoretically twenty times as bright (though practically much of the light is lost by reflection from and passage through the object glass). The magnifying power depends on the relative focal length (the distance from the lens to the image) of the object glass and the eye-glass (the lens next the eye). For example, if the focal length of the object glass be twenty-five inches, and that of the eye-glass one-half inch, then the magnifying power is represented by the quotient of twenty-five by one-half, which is fifty. In order to obtain a good image the illuminating power must be good and the magnifying glass (eye-piece) perform its work well.
Since the time of Galileo, refracting telescopes have been of course much improved. Their size also has increased until they are now constructed with object glasses twenty-six inches in diameter; and it is reported that one is projected for the Lick Observatory in California, which is to have an objective thirty-six inches in diameter. Reflecting telescopes, so called because instead of refracting or bending the rays of light, they reflect them from a concave mirror, have been constructed with circular mirrors six feet in diameter, and it is believed by some makers that they can be successfully operated when the mirrors are as much as seven or eight feet in diameter.
It must not be supposed, however, that in general the greater the telescope, the more successful the observations. Both calculation and experience indicate a limit beyond which increase in size, even if it affords greater power, diminishes in clearness. And it is of little value to us that we bring the moon apparently within forty miles, if we can not distinguish its features—if the face of the “man in the moon” should be a mere blur, like a blot on a piece of paper. It is, in fact, exceedingly doubtful whether the moon has ever been seen through the telescope so well as it would be seen with the naked eye at a distance of 500 miles.
To afford an idea of what has been done in the manufacture of these instruments we mention the following: Of refractors, the one in the United States Naval Observatory at Washington has an object glass twenty-six inches in diameter; that of Mr. R. S. Newall, Gateshead, England, an object glass of twenty-five inches; Observatory of Harvard College, Cambridge, Mass., fifteen inches; Allegheny Observatory, connected with the Western University of Pennsylvania, at Allegheny, Pa., thirteen inches; Michigan University, at Ann Arbor, Mich., twelve and five-tenths inches; Middletown University, Connecticut, eleven inches. Of reflectors, the one constructed by the Earl of Rosse, at Parsontown, Ireland, has a mirror six feet in diameter; the Observatory of Melbourne, Australia, four feet; and Mr. S. Lassell, Marblehead, England, two feet in diameter.