Another mode which I have used for determining, to a near approximation, the powers of telescopes, is as follows:--Endeavour to find the focus of a single lens which is exactly equivalent to the magnifying power of the eye-piece, whether the Huygenian or the common terrestrial eye-piece. This may be done by taking a small lens, and using it as an object-glass to the eye-piece. Looking through the eye-piece to a window and holding the lens at a proper distance, observe whether the image of one of the panes exactly coincides with the pane, as seen by the naked eye; if it does, then the magnifying power of the eye-piece is equal to that of the lens. If the lens be 1/2 inch focal length, the eye-piece will produce the same magnifying power, as a single lens when used as an eye-glass to the telescope, and the magnifying power will then be found by dividing the focal distance of the object-glass by that of the eye-glass. But if the image of the pane of glass does not exactly coincide with the pane as seen by the other eye, then proportional parts may be taken by observing the divisions of such a board as described above, or we may try lenses of different focal distances. Suppose, for example, that a lens 2 inches focal length had been used, and that the image of a pane covered exactly the space of two panes, the power of the eye-piece is then equal to that of a single lens 1 inch focal distance. The following is another mode depending on the same general principle. If a slip of writing-paper one inch long, or a disk of the same material of one inch diameter, be placed on a black ground at from 30 to 50 yards distance from the object-end of the telescope, and a staff painted white, and divided into inches and parts by strong black lines, be placed vertically near the said paper or disk; the eye that is directed through the telescope when adjusted for vision, will see the magnified disk, and the other eye, looking along the outside of the telescope, will observe the number of inches and parts that the disk projected on it will just cover, and as many inches as are thus covered will indicate the magnifying power of the telescope--at the distance for which it is adjusted for distinct vision. The solar power, or powers for very distant objects, may be obtained by the following proportion:--As the terrestrial focal length, at the given distance: is to the solar focal length :: so is the terrestrial power, to the solar power. For example, a disk of white paper one inch in diameter, was placed on a black board, and suspended on a wall contiguous to a vertical black staff that was graduated into inches by strong white lines, at a distance of 33 yards 2-1/2 feet, and when the adjustment for vision was made with a 42 inch telescope, the left eye of the observer viewed the disk projected on the staff, while the right eye observed that the enlarged image of the disk covered just 58-1/2 inches on the staff, which number was the measure of the magnifying power,