Next take two pieces of wood, fifteen inches square and half an inch thick; in the center of one cut a hole twelve inches in diameter, while in that of the other cut another round hole only ten inches across. In Fig. 4, which represents a section of this microscope, g is the central board of the screen, marked a in Fig. 1; h is the piece you have just made, with the central opening twelve inches in diameter; and i is the second piece, which measures fifteen inches square, but has a hole of only ten inches diameter in its center; d, which has a dotted surface, to distinguish it from the frame-work, is the large disk (Fig. 2), which, you remember, is just twelve inches in diameter, and, consequently, will exactly fit the opening in h; if these edges are rough, sand-paper both with a coarse quality first, finishing them off with a finer kind. When d is in position, and moves easily but not loosely in h, place i over it and fasten it in place with screws, passing through i into h; but h, of course, must first be strongly nailed or screwed upon g.
You will now see that by turning the handle, f, the position of the mirror, which is fastened to this disk, can be easily changed so that it shall face in any direction, while by drawing the wire, e, it can be turned so as to reflect the sun’s rays through the lens, a, from whatever quarter of the heavens it may be shining. This double adjustment of the mirror and lens enables you to throw the rays of the sun through the opening in upon the object, o, at any hour of the day. As the mirror is adjusted in Fig. 4, the sun must be very low, as its rays, to strike the mirror, would necessarily be nearly horizontal.
The lens, a, should be held in place by two pieces of whalebone, bent around on either side of it, at the edge of the opening in d; this lens is four inches in diameter, and has a focal length of nine or ten inches; its adjustment had better be left till everything else in the microscope has been finished. Fig. 5 shows the appearance of this when completed.
As the outer part of the work is fitted, we will now turn our attention to the other, or inner, side of the screen. In Fig. 4, the board j is eighteen inches long by ten inches wide, and half-an-inch thick. In the middle is a small circular hole, one and one-half inches in diameter. This is fastened to the middle board, g, by the four horizontal posts, p, each six inches long.
Now take a square piece of half-inch board, five inches across, cut a circular hole two inches in diameter in the middle, and fit into this hole a pasteboard tube four inches long, which is painted black on the inside. The edge of the circular hole in j should also be black. In Fig. 6, which represents this board, you will notice two cleats, l l, fastened to the back of k; these are also made of half-inch wood, and are five inches long by one wide. In Fig. 4, the position of k and l is seen in connection with the longer piece, j; the center of the openings in j and k should form one and the same horizontal line. The opening between l and l is for the glass slides upon which the objects to be examined are placed.
After these parts are fastened in their proper places, make a pasteboard tube, with a black inner surface, as represented at n, about four inches in length, and inclose in one end two lenses, each one and one-half inches in diameter, and each having a focal length of four inches. Fit this tube in the one marked m. Now, having everything in place, fit in the lens, a, so that it will send the rays of light directly through the hole in j upon the object in l, and fasten it securely in place with your bent whalebones.
The screen upon which the image is thrown can be the opposite whitewashed surface of the room, if by a proper adjustment of the tubes the image can be made distinct, or it can be a sheet stretched over a frame-work of light wood; the latter is preferable, as it can be more easily brought in focus. Of course, in this form, as in any other “magic lantern,” the nearer the screen to the lantern, the longer the tubes m n; but the image, which is smaller, gains in brilliancy of illumination, while with these conditions reversed, the results are the opposite; a larger image, but less bright in appearance. The same light being spread over a larger surface is necessarily less strong.