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.