[Page 43] III.

THE TELESCOPE.

Frequent allusion has been made in the previous chapter to discovered results.  It is necessary to understand more clearly the process by which such results have been obtained.  Some astronomical instruments are of the simplest character, some most delicate and complex.  When a man smokes a piece of glass, in order to see an eclipse of the sun, he makes a simple instrument.  Ferguson, lying on his back and slipping beads on a string at a certain distance above his eye, measured the relative distances of the stars.  The use of more complex instruments commenced when Galileo applied the telescope to the heavens.  He cannot be said to have invented the telescope, but he certainly constructed his own without a pattern, and used it to good purpose.  It consists of a lens, O B (Fig. 13), which acts as a multiple prism to bend all the rays to one point at R. Place the eye there, and it receives as much light as if it were as large as the lens O B. The rays, however, are convergent, and the point difficult to [Page 44] find.  Hence there is placed at R a concave lens, passing through which the rays emerge in parallel lines, and are received by the eye.  Opera-glasses are made upon precisely this principle to-day, because they can be made conveniently short.

[Illustration:  Fig. 13.—­Refracting Telescope.]

If, instead of a concave lens at R, converting the converging rays into parallel ones, we place a convex or magnifying lens, the minute image is enlarged as much as an object seems diminished when the telescope is reversed.  This is the grand principle of the refracting telescope.  Difficulties innumerable arise as we attempt to enlarge the instruments.  These have been overcome, one after another, until it is now felt that the best modern telescope, with an object lens of twenty-six inches, has fully reached the limit of optical power.

The Reflecting Telescope.

This is the only kind of instrument differing radically from the refracting one already described.  It receives the light in a concave mirror, M (Fig. 14), which reflects it to the focus F, producing the same result as the lens of the refracting telescope.  Here a mirror may be placed obliquely, reflecting the image at right angles to the eye, outside the tube, in which case it is called the Newtonian telescope; or a mirror at R may be placed perpendicularly, and send the rays through [Page 45] an opening in the mirror at M. This form is called the Gregorian telescope.  Or the mirror M may be slightly inclined to the coming rays, so as to bring the point F entirely outside the tube, in which case it is called the Herschelian telescope.  In either case the image may be magnified, as in the refracting telescope.

[Illustration:  Fig. 14.—­Reflecting Telescope.]

Reflecting telescopes are made of all sizes, up to the Cyclopean eye of the one constructed by Lord Rosse, which is six feet in diameter.  The form of instrument to be preferred depends on the use to which it is to be put.  The loss of light in passing through glass lenses is about two-tenths.  The loss by reflection is often one-half.  In view of this peculiarity and many others, it is held that a twenty-six-inch refractor is fully equal to any six-foot reflector.