and breadth. This forms what is called an Astroño mical telescope; but, as every thing seen through it appears inverted, it is not adapted for viewing terrestrial objects. In order to fit it for viewing land objects, two other eye-glasses, of the same focal distance, (namely, one inch,) are requisite; the second eyeglass IK is placed at 2 inches from EF, or double their focal distance, and the glass NO at the same dis tance from IK.* By this means a second image IM is formed in an upright position, which is viewed by the eye at P, through the glass NO, and the object appears magnified in the same proportion as before. The magnifying power of a telescope of this construction is found by dividing the focal distance of the object-glass by the focal distance of the eye-glass. Thus, if the object-glass be 36 inches in focal distance, and the eye-glass 11⁄2 inch, the magnifying power will be 24 times; if the focus of the eye-glass be 2 inches the magnifying power will be 18 times, &c.-LM is the telescope fitted up for use. A compound microscope might likewise be easily constructed by any ingenious artizan or mechanic, by attending to the following illustrations and directions. Fig. 2 represents the glasses of a compound microscope. AB is the glass next the object; CD is the amplifying glass for enlarging the field of view; EF is the glass next the eye. When a small object, as GH, is placed below the object-glass AB, at a little more than its focal distance from it, a magnified image of this object is formed by the glass AB at GH, which is magnified in proportion as the distance GG exceeds the distance of AG. This magnified image of the object is magnified a second time by the glass EF, to which the eye is applied at K. This instrument, when fitted up for use, is represented in fig. 3, where LM represents a box or pedestal on which it stands, NO the stage on which the objects are placed, over the opening i, which is supported by *This is not the best form of a terrestrial eye-piece; but it may serve for the purpose of illustration. The eye-piece now most generally used consists of four lenses, combined on a different principle. 3 pillars fixed to the top of the box. P is a tube which is supported by 3 pillars fixed into the stage. Into this tube the tube R slides up and down for adjusting the focus. The small tube u, which car ries the object-glass, is connected with the tube R, and slides up and down along with it. S is the tube which contains the two eye-glasses, and which may be made to slide up and down into the tube R, for increasing the magnifying power when occasion requires. T is a mirror, fixed on the pedestal, capable of moving up and down, and to the right or left, for throwing light upon the object placed over the hole i, which may be laid upon a slip of thin glass. The object-glass AB, fig. 2, is placed at u, fig. 3. The glass CD is placed opposite W, fig. 3, and the eyeglass EF opposite V. Such are the essential parts of a compound microscope. Any common mechanic may construct one for himself by attending to the following directions: The object-glass AB, fig. 2, may be about,, or 1 inch focal distance, and the aperture, or hole which lets in the light from the object, should not exceed 1-10th of an inch, otherwise it will cause a glare, which will produce an indistinct image of the object. The amplifying glass CD may be 24 inches focal distance, and 1 inch in diameter. This glass is not essentially necessary, but it serves to enlarge the field of view, and to render it more distinct near the border. eye-glass EF should be about 1 inch focus, and about The inch in diameter. With respect to the distances at which they should be placed from each other, the glass CD may be placed at about 5 or 6 inches from AB, and the glass EF about 2 inches, or 13 inch from CD. The object-glass should be a double convexthe eye-glasses may be plano-convex; that is, plane on the one side and convex on the other, with the plane sides turned next the eye; but double convexes will do, if these cannot be procured. The tubes which contain the glasses may be made of pasteboard, and the stage, pillars, and box, of wood. The |