Thus we see how, when a small hole is cut in the window shutter of a dark room, a picture of the sun, and bright clouds about it, is formed on the opposite wall. The smaller the hole (so far at least as geometrical optics is concerned) the less con fused will the picture be. As the hole is made larger the illumi nated portions from different sources gradually overlap; and when the hole becomes a window we have no indications of such a picture except from a body (like the sun) much brighter than the other external objects. Here the picture has ceased to be one of the .sun, it is now a picture of the window. But if the wall could be placed ioo m. off, the picture would be one of the sun. To prevent this overlapping of images, and yet to admit a good deal of light, is one main object of the lens which usually forms part of the camera. (See PHOTOGRAPHY : Apparatus.) The formation of pictures of the sun in this way is well seen on a calm sunny day under trees, where the sunlight penetrating through small chinks forms elliptic spots on the ground. During a partial eclipse of the sun, the images formed by such chinks, or by truly plane spots on a glass window pane (which act as chinks) take the form of crescents that are images of the partly obscured disc of the sun. (See also Nature, 89, 1912, p. 216.)
Another curious phenomenon may fitly be referred to in this connection, viz., the phantoms which are seen when we look at two parallel sets of palisades or railings, one behind the other, or look through two parallel sides of a meat-safe formed of per forated zinc. The appearance presented is that of a magnified set of bars or apertures which appear to move rapidly as we slowly walk past. Their origin is the fact that where the bars appear nearly to coincide the apparent gaps bear the greatest ratio to the dark spaces; i.e., these parts of the field are the most highly illu minated. The exact determination of the appearances in any given case is a mere problem of convergents to a continued frac tion. But the fact that the apparent rapidity of motion of this phantom may exceed in any ratio that of the spectator is of impor tance—enabling us to see how velocities, apparently of impossible magnitude, may be accounted for by the mere running along of the condition of visibility among a group of objects no one of which is moving at an extravagant rate.