of the two half-shadows outwards, and the diminution of the principal shadow inwards, both being effects of the cross action of the light.
396.
This experiment may be repeated and varied before a smooth wall, with rods of different thicknesses, and again with balls; we shall always find that the farther the object is removed from the surface of the wall, the more the weak double shadow spreads, and the more the forcible main shadow diminishes, till at last the main shadow appears quite effaced, and even the double shadows become so faint, that they almost disappear; at a still greater distance they are, in fact, imperceptible.
397.
That this is caused by the cross-action of the light we may easily convince ourselves; for the shadow of a pointed object plainly exhibits two points. We must thus never lose sight of the fact that in this case the whole sun-image acts, produces shadows, changes them to double shadows, and finally obliterates them.
398.
Instead of solid bodies let us now take openings cut of various given sizes next each other, and let the sun shine through them on a plane surface at some little distance; we shall find that the bright image produced by the sun on the surface, is larger than the opening; this is because one edge of the sun shines towards the opposite edge of the opening, while the other edge of the disk is excluded on that side. Hence the bright image is more weakly lighted towards the edges.
399.
If we take