142 FATHER SCHEINER. vens, or of the landscape, which could be seen at once, without moving the eye or the instrument, was more extensive; but the objects were still inverted,* which was very inconvenient when the instrument was employed in the examination of objects on the earth. Scheiner considered the matter further, and at last found an expedient which removed that difficulty. In order to avoid the awkwardness of seeing objects upside down, he added a second convex lens to the eye-glass, which, by inverting the image before it reaches the lens nearest to the eye of the observer, causes it, after being, as it were, set upright again, by passing through that lens, to appear in its natural position. It is surprising that forty years should have elapsed from the first invention of telescopes, before any one thought of adopting so simple an expedient.† You will understand it better if I take this pocket-telescope to pieces, and show you the parts, than by seeing lines drawn upon paper. *Hutton, ib. 569. Joyce, Sci. Dial. ii. 218. 1 CONSTRUCTION OF THE TELESCOPE. 143 A telescope is merely a long tube, blackened within to destroy useless light, and fitted up with the lenses I am going to show you. First, in the large end is the object-glass, the window that admits the view: at the opposite end of the small tube that slides within the large one, you see there is another lens: this is the eyeglass; and in telescopes containing only these two lenses, the objects seen are always inverted, as they were in Galileo's instrument. Before we proceed farther, I would observe, that although this telescope consists of four tubes, sliding one within the other, such an arrangement has no other effect than to make the instrument more portable. One slide, indeed, is necessary, because it enables each observer to adjust the focus so as exactly to suit his own eye; but the repetition of slide within slide, like this, is merely for convenience: when the tubes are all slipped back, the telescope may be easily put into the pocket. You may form a good idea of the necessity for Scheiner's first improvement on the Galilean telescope, by looking through the large one on the table, in its present state. You will see the trees in the orchard before us appear beautifully distinct: you may even distinguish the veins in their leaves; and by screwing on this small magnifier in front of the eye-glass, we can increase its power. But the objects are still inverted; and, therefore, a glass of this construction is fit only for viewing the heavenly bodies. It is called an astronomical telescope. When we desire to use the instrument for viewing terrestrial objects, we must take out the eye-glass, and substitute a tube like this. The lenses it contains, restore the objects to their natural position: they are an improvement on the original invention of Father Scheiner. Perhaps it is in consequence of the first, or Galilean telescope, having contained only two lenses, which were called the object-glass and the eye-glass, that when Father Scheiner added a third lens, and soon afterwards Father Reita added a fourth, these were still considered as belonging to the eye-glass; and they do, in fact, DAY TELESCOPE. 145 assist the eye, by restoring the natural position and rendering, more distinct, the image transmitted through the object-glass. Therefore, considering all these three lenses as appropriated to the service of the eye, the telescope is still said to consist of the object-glass and the eye-glass. An instrument of this construction is called a land, or day telescope. It is adapted for viewing objects in the day-time, on or near the earth. It enables the sailor, whose unassisted eye can only perceive a small speck in the horizon, to discover what kind of vessel is just rising above the convexity of the earth. He may distinguish to what nation she belongs; and, by aid of mutual signal-flags, hold converse with the distant crew. And thus a man, in the midst of a solitary plain, on the top of a mountain, or even in his own garden, or near some open window in his house, who supposed himself unseen, might yet, by a telescope, be instantly placed under the observation of any person who chose to watch H 146 DAY TELESCOPE. him. It is said that some remarkable instances have occurred, in which actions, imagined by the parties concerned to have been performed in perfect secrecy, have thus been made known.* Would any person be deterred from unworthy conduct by such a thought? How much more impressive should be the conviction, that we are perpetually beneath the notice of an Eye, which needs no lens to assist its powers, but sees "in all dark places, by night as well as by day!" One of the great difficulties with which the first makers of telescopes had to contend, was occasioned by the separation of the rays of light into the distinct colours of which they are composed. The lens which was so useful in refracting the rays, and converging them to one point, or focus, had also the property of dividing them, like a prism; and, in consequence of this, the image formed behind the lens was surrounded by coloured fringes, which made it Arnott, ii. p. 290. |