magnifying power of such a microscope, we have only to multiply the magnifying power of the object-glass by that of the eye-glass. If, for example, the distance of the image o o from the object-glass L be ten times as great as the distance of the object from it, the linear dimensions of the image will be ten times greater than those of the object; and if the focal length of the eye-glass be half an inch, the distance of most distinct vision being 10 inches, the linear dimensions of o' o' will be twenty times those of o o, and consequently 200 times those of the E Fig. 40. object; the linear magnifying power in such a case would be 200, and the superficial magnifying power 40,000. The distance apart at which the eye-glass and object-glass are usually mounted is about ten or twelve inches, adjustments being provided by which the distance within certain limits can be varied. THE TELESCOPE. 1. The telescope is an optical instrument for viewing distant objects, by increasing the apparent angle under which they are seen without its assistance; and hence the effect is an apparent increase in size, or a magnified representation of the object. The word "telescope" is derived from two Greek words, which signify "at a distance," and "I view." The invention of the telescope is one of the most important acquisitions the sciences ever attained, as it unfolds to our view the wonderful mechanism of the heavens, and enables us to obtain data for astronomical, nautical, and engineering purposes. 2. The principle is identical with that of the compound microscope. An image of the object is produced by means of a concave reflector or a converging lens, and this image is viewed with a microscope composed of one or more converging lenses. Telescopes consist, therefore, of two classes, reflectors and refractors; in the former the image is produced by concave reflectors, and in the latter by lenses. 3. The simplest construction of the telescope consists of two convex lenses so combined as to increase the apparent angle under which distant objects are seen. If we take a convex lens, of say 8 inches' focus, as an object-glass, and another, of say 2 inches' focus, as an eye-glass, and place them at a distance apart equal to the sum of their foci, or 10 inches, we obtain a telescope suitable for viewing distant objects transmitting parallel rays; but when the object is comparatively near, the distance between the two lenses must be increased to adjust for distinct vision; on this account the eye-glass is mounted in a tube, sliding within another tube, in which the object-glass is fixed, and, therefore, can be drawn out for near objects. 4. The common astronomical telescope, shown in Fig. 41, is of the same principle of construction. It consists of two convex lenses A B, C D, the former of which is the object-glass, and the latter the eye-glass, from being near the eye E. mn is an image of any distant object M N. Huygens made a telescope on this principle, of which the focus of the object-glass was 123 feet in length, and the diameter of the aperture 6 inches, the focal length of the eye-glass being 6 inches. With instruments 12 and 24 feet long he discovered the ring and the fourth satellite of Saturn. In order to use object-glasses of great focal length without the incumbrance of long tubes, or incurring cost, Huygens placed the object-glass in a short tube at the top of a very long pole, so that the tube could be turned in any direction upon a ball and socket by means of a cord, and brought into the same line with another short tube containing the eye-glass which he held in his hand. 5. Sir William Herschel, after having constructed several reflecting telescopes varying from 7 to 20 feet in length, completed, in 1789, his 40 feet telescope, by which, on the very day it was finished, he discovered the sixth satellite of Saturn. The great speculum of this telescope measured 4 feet in diameter, and the area of its reflecting surface was consequently 12.566 square feet, its thickness being 3 inches, and its weight 1,050 lbs. The open end of the telescope being directed to that part of the heavens under observation, and the speculum being fixed at its lower end, the observer was suspended in a chair, so as to be able to look over the lowest edge of the opening. As the speculum was a little inclined to the axis of the tube, the image was formed at about 2 inches from the lowest part of the edge of the opening, where it was viewed by the observer with suitable eye-pieces. The great quantity of light obtained by this speculum or mirror enabled its celebrated constructor to use a magnifying power of 6,450 when examining the fixed stars; a power which greatly exceeded any that had been previously employed. 6. The telescope (Fig. 42) was mounted on a platform, which revolved horizontally, on rollers. It was placed between four ladders, which served both as a framework for its support and as a means of reaching the upper end of the great tube. The ladders were united at the upper ends by being bolted to a cross-bar, to which the pulleys were attached. The telescope Fig. 42.-Herschel's Great Telescope. was raised or lowered by one system of pulleys, and the gallery in which the observer stood by another. The pulleys were worked by a windlass placed on the platform. As the frame of this instrument had decayed, it was taken down and another telescope of smaller size erected in its place by Sir J. F. W. Herschel, with which many important observations have been made. 7. The largest and most powerful instrument of celestial investigation ever constructed was made by the late Lord Rosse, at Parsonstown, Ireland (Fig. 43). The clear aperture is six feet in diameter, and consequently the area of its reflecting surface is 28.274 square feet, whilst that of Herschel's great telescope was only 12.566 square feet. The tube is 52 feet in length. This instrument is so constructed that it may be used either as a Newtonian telescope-that is, the rays proceeding along the axis of the great speculum* are received at an angle of 45° upon a *A metal mirror. second small speculum, by which the image is thrown towards the side of the tube, where it is examined by the eye-piece-or otherwise. Two specula have been provided for the telescope, one of which weighs 3 and the other 4 tons, composed of copper and tin, in the proportion of 126 parts, by weight, of the former, to 57 of the latter. When directed towards the south the tube can be lowered until it is nearly horizontal; towards the north it can only be lowered to the altitude of the pole. It is so counterpoised as to be moved with ease in the required direction. 8. "I have enjoyed the great privilege of seeing this noble instrument," says Sir David Brewster, "one of the most wonderful combinations of art and science that the world has yet seen. I have in the morning walked again and again, and ever with new delight, along its mystic tube, and in the evening, with its distinguished inventor, pondered over the marvellous sights which it discloses the satellites, and belts, and rings of Saturn; the old and the new ring, which is advancing with its crest of waters to the body of the planet; the rocks, and mountains, and valleys, and extinct volcanoes of the moon; the crescent of Venus, with its mountainous outline; the system of double and triple stars; the nebulæ and clusters of stars of every variety of shape; and those spiral nebular formations which baffle human comprehension, and constitute the greatest achievement of modern discovery." M ADAM AND EVE'S MORNING HYMN.-John Milton. See "Fifth Reader," p. 215. To the field they haste, But first, from under shady arborous roof Flow'd from their lips, in prose or numerous verse To add more sweetness; and they thus began: Thus wondrous fair: Thyself how wondrous then! In these thy lowest works; yet these declare Him first, him last, him midst, and without end If better thou belong not to the dawn, Sure pledge of day, that crown'st the smiling morn |