Direct and reflected light entering a cavern
Direct light penetrates two-thirds of the cavern, reflected light the remaining third
A study of how light fills an opening: the chamber a b c d, open on two faces, receives direct light two-thirds more powerful than the reflected light r S. The direct light therefore penetrates two-thirds of the cavern while the reflected light takes only the remaining third n d. At the top a labelled diagram shows the cavern as a rectangle set beneath a semicircle representing the external light.
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Direct light outreaches reflected light in a cavern
The cavern a b c d is open on two faces; its direct light is two-thirds more powerful than the reflected light r S. Consequently the direct light penetrates two-thirds of the cavern, and the reflected light r S occupies only the third portion n d.
Labelled diagram of the opening and the semicircle of light
At the head of the page a rectangle marked with the letters of the cavern (a b c d) is set within a semicircular arc standing for the external light, with lettered points r, S, f and others locating the light rays and the divisions of the opening.