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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