Young, George, jr, Baxter, Hubert Eugene, joint author · New York, The Macmillan Co. · leaf 261
we cannot deal extensively with the problem ofintensity of illumination. SufSce it to say that the intensityof illumination will be greatest on those surfaces which arenormal to the rays of light, and zero on those surfaces whichare tangent to the light rays. All conceivable intermediatecases… … Fig. 226 will change its form or its position or both. If the source ismoved to an infinite distance, the tangent cone becomes atangent cylinder, and the rays of light are all parallel. Thisis what happens, in effect, when the sun is takCn as the sourceof light. In order to simplify the problem of…
Geometry, Descriptive
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