It is also to be observed that in this instance again light does not act as an abstract diffusion (361), the sun shines towards an edge. The volume of light poured from the sun-image passes by the edge of a substance, and occasions shadows. Within these shadows we shall presently find colours appear.
But, above all, we should make the experiments and observations that bear upon our present inquiry in the fullest light. We, therefore, place the observer in the open air before we conduct him to the limits of a dark room.
A person walking in sun-shine in a garden, or on any level path, may observe that his shadow only appears sharply defined next the foot on which he rests; farther from this point, especially round the head, it melts away into the bright ground. For as the sun-light proceeds not only from the middle of the sun, but also acts cross-wise from the two extremes of every diameter, an objective parallax takes place which produces a half-shadow on both sides of the object.
If the person walking raises and spreads his hand, he distinctly sees in the shadow of each finger the diverging separation of the two half-shadows outwards, and the diminution of the principal shadow inwards, both being effects of the cross action of the light.
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.