The most simple form of the refracting telescope is made with a lens of any focal length exceeding six inches, placed at one end of a tin or cardboard tube, which must be six inches longer than the focal length of the lens; the tube may be in two parts, sliding one within the other, and when the eye is placed at the other end, an inverted image of the object looked at, is apparent. By using two double-convex lenses, a more perfect simple astronomical telescope is obtained. The object-glass, _i.e._, the lens next the object looked at, must be placed at the end of a tin or pasteboard tube larger than its focus, and the second lens, called the eye-glass, because next the eye, is a smaller tube, termed the eye-tube; and if the focal length of the object-glass is three feet, the eye-glass must have a one-inch focus, and of course the eye-tube and glass must slide freely in the tube containing the object-glass. An object-glass of forty feet focus will admit of an eye-glass of only a four-inch focus, and will, therefore, magnify one hundred and twenty times. A tube of forty feet in length would of course be very troublesome to manage, and therefore it is usual to adopt the plan originally devised by Huygens, viz., that of placing the object-glass in a short tube on the [Page 320] top of a high pole with a ball-and-socket joint, whilst the eye-glass is brought into the same line as the object-glass, and focused with a tube and rack-work properly supported. In an ordinary terrestrial telescope there are four lenses, in order that the objects seen by its assistance shall not be inverted; and whenever objects are examined by a common telescope, they are found to be fringed, or surrounded with prismatic colours. This disagreeable effect is corrected by the use of _achromatic_ lenses, in which two kinds of glass are united; and the light decomposed by one glass, uniting with the colours produced by the other form white light, thus a double convex lens of crown glass, C C, may be united with a plano-convex lens of flint glass, F F, which must have a focus about double the length of that of the crown-glass lens. The concave lens corrects the colour or chromatic aberration of the other, and leaves about one-half of the refracting power of the convex lens as the effective magnifying power of the compound lens. The French opticians cement the lenses very neatly together, and use them in ordinary spy and opera glasses. (Fig. 307.) [Illustration: Fig. 307. A compound achromatic lens, composed of C C, the double-convex lens of crown-glass, and F F, the plano-concave lens of flint-glass.] XIV. _The Stereoscope._