Lastly, it is necessary to provide a finder for this camera in order to know what picture you are taking.  Make a little frame of wire, the size of the plate you are using, and mount it upright (see Fig. 5) on top of the camera as close to the end where the pin hole is as you can.  At the other end, in the center, erect a little pole of wire half the height of the plate.  If now you look along the top of this little pole, through the wire frame and see that the top of the little pole appears in the center of the frame, everything that you see beyond will be

[Illustration:  Pin Hole and Shutter Construction]

taken on the plate, as will be made plain by looking at the dotted lines in Fig. 5, which represents the outer limits of your vision when confined within the little frame.

[Illustration:  Explanation of Action of Pin Hole]

When you want to use this camera, take it into an absolutely dark room

[Illustration:  Constructing a Finder for Camera]

and insert a plate (which you can buy at any supply store for photographers) in the end where the slides of wood are, and between them and the back of the box.  Close the lid and secure it with a couple of rubber bands.  See that the little shutter covers the hole.  Now take the camera to where you wish to take a photograph, and rest it securely on some solid surface.  The exposure will be, in bright sunlight and supposing that your camera is 10 in. long, about six to eight seconds.  This exposure is made by lifting the little brass shutter until the hole is uncovered, keeping it up the required time, and then letting it drop back into place.  It is important that the camera be held rigid during the exposure, and that it does not move and is not jarred—­otherwise the picture will be blurred.  Remove the plate in the dark room and pack it carefully in a pasteboard box and several wrappings of paper to protect it absolutely from the light.  It is now ready to be carried to some one who knows how to do developing and printing.

To explain the action of the pin hole I would direct attention to Fig. 2.  Here F represents the front of the camera, D the pinhole, AA the plate and the letters RR, rays from a lighted candle.  These rays of course, radiate in all directions, an infinite multitude of them.  Similar rays radiate from every point of the object, from light reflected from these points.  Certain of these rays strike the pin hole in the front of the camera, represented here by RRRR.  These rays pass through the pin hole, and as light travels only in straight lines, reach the plate AA, forming an inverted image of the object, in this case a candle in a candlestick.  Millions of rays are given off by every point in every object which is lighted by either direct or reflected light.  To all practical purposes only one of these rays from each point in an object can pass through a minute opening like a pin hole.  This being so, any screen