in the shutter. There will then be formed upon the white surface an inverted picture of the external objects, as shown in Fig. 3. They will appear of the natural colors, and the outlines will be sharper in proportion as the hole is smaller.
When the sun shines through a small orifice into a darkened room, a cone of rays is produced, as everybody has observed, by lighting up the particles of dust which are scattered in its course; for, if the air were quite clear, the track of the rays would be invisible. In this case an image of the sun is formed upon the floor or opposite wall by
the crossing of the rays through the aperture, exactly in the manner of the production of the candle-image and the landscape-picture just described. The best condition for the formation of such an image is when the sun is low, and there is a white wall opposite to receive it; the image is then perfectly circular. But if the light falls upon the floor, as represented in Fig. 4, the cone of rays produces an oblong or elliptical image; the deviation from an exact circle depends upon the angle which the cone of rays makes with the floor. Such an image