Telescope

In the earliest, or Galilean, telescope, the eye-lens is concave; a construction only now used In opera-glasses. It has far less chromatic and spherical aberration than the common astronomical telescope, and is shorter, since the distance between the lenses is the difference, not the sum of their focal lengths; but it has a very serious defect in the smallness of its field of view. This can only be enlarged, as in opera-glasses, by mak ing the diameter of the object-lens disproportionately great.

Defore the discovery of the possibility of forming an achromatic lens, Huygens, Cassini, and others, had endeavored, by enormously increasing the focal length of the object-glass of the common astronomical telescope in proportion to its diameter, to get rid as far as possible of chromatic aberration. This was called the aerial telescope, as the object and eye-lenses were mounted separately on stands; the tube (which would have been 100 200, or even 600 ft. long) being dispensed with. Valuable work was done with some of these telescopes, of 125 ft. focus, but the longer ones proved unmanagea ble. The principle involved in these constructions is, practically, the throwing the magnifying power more on the object-lens than on the eye-lens; as the image formed by the former was still so imperfect as not to bear much additional magnification; although achromatic eye-pieces could even then be made with one kind of glass. The great step required for shortening the unwieldy instrument was therefore the perfecting of the object-lens. We have already seen how this was effected. Various very ingenious immvements on achromatic combinations, even yet (we should be inclined to think) worthy the consideration of opticians, were devised by Dr. Blair. He obtained in solutions of mercury or antimony in hydrochloric acid, media, in which. while much more refractive and more dispersive than crown-glass, no irrationality of dispersion as compared with crown-glass could be detected. With these fluid lenses be was enabled to give the telescope an aperture of of its focal length without a trace of residual color. The dialytic telescope, invented in 1828 by Mr. Rodger, and since made by Plossl, seems to promise very well. Its object is to obtain a large aperture for the telescope with a flint-lens (the obtaining of which, in large and perfect disks, is the great difficulty) of moderate size. In this telescope the object-lens is single, and of crown-glass; having, of course, all the defects of the single lens. These are corrected, at some distance in the cone of converging rays, by the interposition of a combination of a pair of much smaller lenses, whose focal lengths are equal for red rays; the first being a convex lens of crown, the second a concave of flint glass. The adjustments of this instrument for exact correction are, a motion of the pair of lenses to or from the object-lens, to correct chromatic aberration ; and a change of the distance between the two smaller lenses, to correct spherical aberration.

Chromatic being so much more serious than spherical aberration, it is not to be ; wondered at that the idea of substituting an object-mirror, in which the former is absent, I for an object-lens, was early suggested. The first practicable scheme for the purpose seems to have been that of Gregory; in which, however, two mirrors are employed. In the skillful hands of Short, this instrument completely superseded the ordinary astro nomical telescope. Its chief defects are, the great loss of light by two direct reflections, and the increase of the spherical aberration by the fact that both mirrors are concave. The first defect is incurable, the second was partially overcome by Cassegrain's plan of using a small convex mirror for the second reflection. To Newton is due the simple idea of using the combination of a single curved mirror with a plane mirror and an eye-piece; a construction differing only in slight particulars from that now universally adopted for reflecting instruments. Newton constructed several such telescopes with his own hands, some of which are still preserved, as in the appartments of the royal society at Bur lington House.

The elder Herschel constructed for himself all the instruments, gradually increasing in magnitude the optical power, by means of which he made his grand discoveries; and his son worthily succeeded him, both as constructor and observer.

The gigantic telescopes of lord Rosse and Mr. Lassell are wonderful examples of delicate art, and have had their full share, with the large achromatics of the present day, in the startling discoveries of modern astronomy.

The process of Liebig for depositing on glass an exceedingly thin film of silver, which, by careful polishing, can be rendered more highly reflective than any other material, has been taken advantage of by Steinheil in the construction of large specula. This is an immense step, since any disk of glass will do, its optical properties not being employed; while, if it be once brought to a true parabolic figure, the silvering may be renewed as often as may be required. One of the great difficulties iu the construction and working of large reflectors has hitherto been the casting and annealing of metallic masses of a few tons' weight. This, in the silvered specula, is entirely avoided. We cannot here enter into a description of the processes, often. extremely ingenious, which have been devised for the grinding, figuring, and polishing of lenses and specula. The reader who desires strictly scientific informatiob, conveyed in a thoroughly popular form, on this and all other points connected with the subject, is referred to sir John Herschel's article, " Telescope," in the Eneyelopteclia Britannica, to which we have been largely indebted, and which has been reprinted in a separate form.