Abstract
WHEN a pencil of light proceeding from a luminous point is incident upon a prism, the rays after refraction do not as a rule diverge from a point, but from two short lines at right angles to each other at some distance apart depending on the angle of incidence of the pencil. These lines are known as the focal lines of the pencil. If the edge of the prism be vertical and the axis of the pencil He in a horizontal plane, the focal lines are respectively horizontal and vertical. The position of the horizontal line is independent of the angle of incidence of the pencil, its distance from the prism being the same as that of the luminous point, or with the notation of Parkinson's “Optics”(p. 88)—
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SHAW, W. Focal Lines . Nature 31, 185–186 (1884). https://doi.org/10.1038/031185a0
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DOI: https://doi.org/10.1038/031185a0