Excerpt from A Dictionary of Photography
There are es1tain forms of reﬂecting and refracting surfaces and lenses in which a particular pencil is reﬂected or refracted without aberration. The cases with which the photographer is chieﬂy con cerned are those of the parabolic and spheroidal mirrors. All rays incident on the surface of a parabolic reﬂector, in directions parallel to its axis, are reﬂected in directions which pass through its focus and all rays incident on the surface of a prolate spheroid, in direc tions which pass through one of its foci, are reﬂected in directions which pass through the other focus.
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