When describing the common refracting telescope, (p. 228.) I have noticed that three eye-glasses, placed at double their focal distances from each other, formerly constituted the terrestrial eye-piece, as represented in fig. 47. But this construction, especially for achromatic instruments, has now become obsolete, and is never used, except in small pocket spy-glasses formed with a single object lens. In its place a four glassed eye-piece has been substituted, which is now universally used in all good telescopes, and which, besides improving the vision and producing an erect position of the images of objects, presents a considerably larger field of view. During the progressive stages of improvement made in the construction of erect eye-pieces by Dollond and Ramsden, three, four, and five lenses were successively introduced; and hence, in some of the old telescopes constructed by these artists, we frequently find five lenses of different descriptions composing the eye-piece. But four lenses, arranged in the manner I am now about to describe, have ultimately obtained the preference. In a telescope having a celestial eye-piece of the Huygenian form, the image that is formed in the focus of the object glass, is that which is seen magnified, and in an inverted position; but when a four glassed eye-piece is used, which produces an erect view of the object, the image is repeated, and the _second_ image, which is formed by the inner pair of lenses AB on an enlarged scale, is that which the pair of lenses CD at the eye-end render visible on a scale still more enlarged. The modern terrestrial eye-piece, represented in fig. 79, is, in fact, nothing else than a compound microscope, consisting of an object lens, an amplifying lens, and an eye-piece composed of a pair of lenses on the principle of the Huygenian eye-piece. Its properties will be best understood by considering the first image of an object, which is formed in the focus of the object glass, as a small luminous object to be rendered visible, in a magnified state, by a compound microscope. The object to be magnified may be considered as placed near the point A, and the magnified image at _i_, which is viewed by the lens D. Hence, if we look through such an eye-piece at a small object placed very near the lens A, we shall find that it acts as a compound microscope of a moderate magnifying power increasing, in some cases, the diameter of the object about 10 times, and 100 times in surface. [Illustration: _figure 79._]