The following sections of this BookRags Literature Study Guide is offprint from Gale's For Students Series: Presenting Analysis, Context, and Criticism on Commonly Studied Works: Introduction, Author Biography, Plot Summary, Characters, Themes, Style, Historical Context, Critical Overview, Criticism and Critical Essays, Media Adaptations, Topics for Further Study, Compare & Contrast, What Do I Read Next?, For Further Study, and Sources.
(c)1998-2002; (c)2002 by Gale. Gale is an imprint of The Gale Group, Inc., a division of Thomson Learning, Inc. Gale and Design and Thomson Learning are trademarks used herein under license.
The following sections, if they exist, are offprint from Beacham's Encyclopedia of Popular Fiction: "Social Concerns", "Thematic Overview", "Techniques", "Literary Precedents", "Key Questions", "Related Titles", "Adaptations", "Related Web Sites". (c)1994-2005, by Walton Beacham.
The following sections, if they exist, are offprint from Beacham's Guide to Literature for Young Adults: "About the Author", "Overview", "Setting", "Literary Qualities", "Social Sensitivity", "Topics for Discussion", "Ideas for Reports and Papers". (c)1994-2005, by Walton Beacham.
All other sections in this Literature Study Guide are owned and copyrighted by BookRags, Inc.
Searchlights depend on special lenses and reflectors to focus electric light into a pinpoint beam that can illuminate objects thousands of feet away. Since about 1870, carbon arc lamps have been used as the light source for searchlights. During World War I, Elmer Sperry, an American engineer, invented a high-intensity arc searchlight. The United States Navy and other armed forces quickly adopted Sperry's light for military purposes.
In today's large searchlights, chemicals are added to the carbon to increase the arc light's brilliance. Like a car's headlights, searchlights focus their beam with a parabolic reflector, a curved metal cup which has the special property that it directs the light scattering from the source into a narrow stream of parallel rays. Parabolic reflectors came into use in the late 1800s. Before then, searchlights had used a special mirror invented by Colonel Alphonse Mangin for the French army in 1877. Some searchlights also use a Fresnel lens to concentrate the light beam. This type of lens, which has a surface divided into concentric rings, was originally developed in 1820 for lighthouses by the French physicist Augustin-Jean Fresnel (1788-1827).