The Solar Ray Through an Aperture: How Light Changes
Geometry of sunlight cast through a small hole into a dark room
A large geometric diagram traces rays of sunlight entering a dark room through a narrow aperture, with the sun drawn as a disc at right and the beam fanning through the opening into overlapping shaded ellipses at left. An accompanying note states that a solar ray passing through the apertures of houses changes in both quality and quantity at every degree of its length. A lettered demonstration (a b c d e f g, r s and a numbered series of figures) analyses how much of the sun's body is 'seen' at each point, grading the brightness by means of the triangles c e m, d e l and e g K. A smaller architectural sketch in perspective appears at lower right, and further untranscribed jottings are present on the sheet.
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Solar ray changing quality and quantity along its length
A ray of the sun that penetrates through the apertures of houses changes in both quality and quantity at every degree of its length. The same holds when a solar ray passes through a narrow aperture made in a thin wall into a dark place.
Grading brightness by triangles c e m, d e l and e g K
The graded figure uses the triangles c e m, d e l and e g K to measure the light. The triangle c e m carries the whole disc of the sun to m, giving the first degree of brightness there, while d e l brings only half the light to l because only half the sun shows itself. In the triangle e g K the full brightness of the sky e g is carried to K.
The sixth demonstration: the sun seen whole versus by half
In the sixth demonstration the sun is stronger at a b than at c d, because a b sees and is seen by the whole body of the sun whereas c d is seen by only half of it. The analysis treats how much of the solar disc is visible at each point behind the aperture.