Supposing that an achromatic telescope of 3-1/2 feet focal length, and 3-1/4 inches aperture were offered for sale, and that it were required to ascertain whether the object-glass, on which its excellence chiefly depends--is a good one and duly adjusted;--some opinion may be formed by laying the tube of the telescope in a horizontal position, on a firm support, about the height of the eye,--and by placing a printed card or a watch glass vertically, but in an inverted position, against some wall or pillar, at 40 or 50 yards distant, so as to be exposed to a clear sky. When the telescope is directed to this object, and accurately adjusted to the eye--should the letters on the card, or the strokes and dots on the watch-glass appear clearly and sharply defined, without any mistiness or coloration, and if very small points appear well defined--great hopes may be entertained that the glass will turn out a good one. But a telescope may appear a good one, when viewing common terrestrial objects, to eyes unaccustomed to discriminate deviations from perfect vision, while it may turn out to be an indifferent one, when directed to certain celestial objects. Instead therefore of a printed card, fix a black board, or one half of a sheet of black paper, in a vertical position at the same distance, and a circular disk of white writing paper, about 1/4 of an inch in diameter, on the centre of the black ground. Then having directed the telescope to this object, and adjusted for the place of distinct vision, mark with a black-lead pencil the sliding eye-tube, at the end of the main tube, so that this position can always be known; and if this sliding tube be gradually drawn out, or pushed in, while the eye beholds the disk, it will gradually enlarge and lose its colour, till its edges cease to be well-defined. Now, if the enlarged misty circle is observed to be concentric with the disk itself, the object-glass is properly centered, as it has reference to the tube; but if the misty circle goes to one side of the disk, the cell of the object-glass is not at right angles to the tube, and must have its screws removed and its holes elongated, by a rattailed file, small enough to enter the holes. When this has been done, the cell may be replaced, and the disk examined a second time, and a slight stroke on one edge of the cell, by a wooden mallet, will show by the alteration made in the position of the misty portion of the disk, how the adjustment is to be effected, which is known to be right when a motion in the sliding tube will make the diluted disk enlarge in a circle concentric with the disk itself. When the disk will enlarge so as to make a ring of diluted white light round its circumference, as the sliding tube holding the eye-piece is pushed in or drawn out, the cell may be finally fixed by the screws passing through its elongated holes.