In the diagram, the dotted lines show the course of the light from the object in view, through the lens (where the rays cross each other) to the looking-glass, and thence to the ground glass above.
A simpler form of the camera obscura is seen in Fig. 2. Here the case is a small soap or spice box, the lens a convex spectacle-glass, and the board marked _b_ a partition, serving as a screen upon which the image is thrown. In this form the lens may be fixed in the end of the box if desired, which is much easier than adjusting it in a sliding tube. The focal distance of an ordinary spectacle-glass averages about twelve or fourteen inches, and the box should be, of course, somewhat longer than the focal length of the glass used. A glass from “near-sighted” spectacles will not do, as it is concave instead of convex.
The box is now pointed out of the window, at some well-marked object, such as a sun-lighted building, and the partition-board moved backward and forward, till the point is found at which the image on it is best defined. Then the board may be fastened (as _a_, _b_, _c_, _d_, Fig. 2) and the top put on, of which the end should be cut off about four inches from the screen, as shown in the figure.
A piece of black cloth thrown over the head, and completely covering the ends of the opening, renders the image more distinct.
This form of camera may be also used for sketching from nature, by raising it on end, and providing it with an inclined mirror, as shown in Fig. 4. The opening _a_, _b_, _c_, _d_, should in this case be sufficiently large to admit easy play of the hand in sketching, and also allow an unobstructed view of the image. The mirror may be prepared without much difficulty. Get the glazier to cut for you a piece of looking-glass three or four inches square, and cover the back with a piece of thick paper or card, to prevent scratching; then take a wire of sufficient length, and double it as in _A_; now bend this double wire in the form of _B_, letting the ends come at _a_ and _b_, and placing your mirror, face downward, upon the frame as in _C_; hold it in place by two bands of strong paper, passed around the glass and wires, and pasted strongly on the back, at the top and bottom of the mirror. When in use this frame is placed over the lens, and reflects the image down upon the drawing-paper placed on the screen below. The four round-headed screws in the top are intended for attaching an opaque curtain to the box, which, however, is only needed when the artist is working in the open air. Then the dark curtain is buttoned in place, and falls over the head and shoulders, completely shielding the image from any invading rays of light which might otherwise confuse the draughtsman.