Di-op′tric light.
The dioptric system of lighting, used in lighthouses, as distinguished from the catoptric, which is by reflectors.
Refraction instead of reflection.
Lenses were used in the South Foreland light in 1752, and in the
Portland light,
England, in 1789.
The system fell into disfavor, owing to certain mechanical difficulties in the construction and arrangement of the lenses.
It was revived and improved by Fresnel about 1810, and has been generally adopted throughout
France and
Holland, and partially in
England.
It is considered superior to the catoptric, and was readopted in
England in 1834, being placed in the
Lundy Island Lighthouse,
Devonshire, England.
The Fresnel dioptric lamp consists of a mechanical, four-wicked oil-lamp, placed in the center of an octagonal glass prism; the center part of each of the sides being formed of a plano-convex lens of about 15 inches diameter, which is surrounded by a series of glass rings of a spherical triangular form, so as to produce the same effect upon the rays as is produced by the central lens.
Allan Stevenson,
Arago, and
Faraday are credited with improvements in the details.
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Fresnel's dioptric light. |
The flame is placed in the focus of the lenses, and the beams are bent parallel to each other, so as to form a solid beam of light proceeding from each lens.
The lenses, after careful and persistent attempts, were merged into a cylindrical hoop which formed the central zone around the flame, as seen in the elevation
a. The rays striking above and below were bent so as to assume a position parallel to those proceeding from the hoop, as seen in the section
b.
