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Ramsden’s dioptric micrometer consists of a divided lens placed in the conjugate focus of the innermost lens of the erecting eye-tube of a terrestrial telescope.

The light is of the dioptric kind—bright, steady, and uniform, and when the weather is too foggy to allow it to be seen, a bell is tolled by machinery, to give the needful warning.

A dioptric telescope, fitted with two tubes joining, so as to enable a person to view an object with both eyes at once; a doubleÏbarreled field glass or an opera glass.

During his residence in Birmingham, Messrs Chance being makers of glass for use in lighthouse lamps, his attention was naturally turned to problems of lighthouse illumination, and he was able to devise improvements in both the catoptric and dioptric methods for concentrating and directing the beam.

Holophote, hol′o-fōt, n. an improved optical apparatus now used in lighthouses, by which all the light from the lamp is thrown in the required direction, in the catoptric holophote by reflectors, in the dioptric by refracting lenses, in the catadioptric by both combined.—adj.