In constructing a telescope similar to the one described above, the object-glass ought to be of considerable diameter and of long focus; the eye-piece, on the contrary, should be comparatively small and of short focus. A little consideration will show the reason of this. An object-glass of long focus will form a large image at the point _a b_, and the eye-piece of short focus will magnify this image more than another lens of less convexity. It is, however, on the size, length of focus, and perfection of workmanship of the objective that the excellence of the telescope depends; large object-glasses are consequently rare, and are only to be found in observatories of the first class. The object-glass of the large telescope at the Observatory at Paris is nearly fifteen inches in diameter, and the highest magnifying power capable of being employed with it is 3,000. The Observatory of Pulkowa, near St. Petersburg, possesses a similar instrument, and the Observatory at Chicago, United States, a still larger one, measuring between eighteen and nineteen inches in diameter. But the largest of all is an objective in the possession of Mr. Buckingham, an amateur astronomer, who has an observatory near London, which is twenty inches in diameter, and twenty-eight feet in focal length. The eye-pieces of astronomical telescopes are of different powers, and are changed according to the class of object to be observed. Thus, in taking a general view of the moon, a low power would be used. If you wished to examine any particular mountain, you would raise the magnifying power by inserting a stronger eye-piece. The power used also depends on the state of the atmosphere. For instance, on warm evenings, when the air is charged with moisture, the tremulousness of the atmosphere is so great, that it is often only possible to use the very lowest power. By combining four convex lenses together, we obtain what is called a terrestrial or erecting eye-piece, which has the property of _re-reversing_ the image formed by the objective. The eye-pieces of all telescopes for use on land or at sea are made on this principle. The same effect may be obtained, as we have already shown in fig. 41, by using a concave lens, but in this the field of view is much diminished.