The magnifying power of this telescope is determined precisely in the same way as that of the astronomical telescope. Suppose the object-glass to be thirty inches focal distance, and each of the eye-glasses 1-1/2 inch focal distance, the magnifying power is in the proportion of 30 to 1-1/2, or 20 times, and the instrument is, of course, considerably longer than an astronomical telescope of the same power. The distance, in this case, between the object-glass and the first eye-glass EE is 31-1/2 inches; the distance between EE, and the second glass FF, is 3 inches, and the distance between FF and the glass GG next the eye, 3 inches; in all 37-1/2 inches, the whole length of the telescope. Although it is usual to make use of three eye-glasses in this telescope, yet two will cause the object to appear erect, and of the same magnitude. For suppose the middle lens FF taken away, if the first lens EE be placed at X, which is double its focal distance from the image IM, it will at the same distance X _m_, on the other side, form a secondary image _i m_ equal to the primary image IM, and also in a contrary position. But such a combination of eye-glasses produces a great degree of colouring in the image, and therefore is seldom used. Even the combination now described, consisting of three lenses of equal focal distances, is now almost obsolete, and has given place to a much better arrangement consisting of _four_ glasses, of different focal distances--which shall be afterwards described.