We might rather consider the splendour of the sun, or of any light, as an infinite specular multiplication of the circumscribed luminous image, whence it may be explained that all square openings through which the sun shines, at certain distances, according as the apertures are greater or smaller, must give a round image of light.
403.
The above experiments may be repeated through openings of various shapes and sizes, and the same effect will always take place at proportionate distances. In all these cases, however, we may still observe that in a full light and while the sun merely shines past an edge, no colour is apparent.
404.
We therefore proceed to experiments with a subdued light, which is essential to the appearance of colour. Let a small opening be made in the window-shutter of a dark room; let the crossing sun-light which enters, be received on a surface of white paper, and we shall find that the smaller the opening is, the dimmer the light image will be. This is quite obvious, because the paper does not receive light from the whole sun, but partially from single points of its disk.
405.
If we look attentively at this dim image of the sun, we find it still dimmer towards the outlines where a yellow border is perceptible. The colour is still more apparent if a vapour or a transparent cloud passes before the sun, thus subduing and dimming its brightness. The halo on the wall, the effect of the decreasing brightness of a light placed near it, is here forced on our recollection. (88.)
406.