The Third and a half Order Fresnel Lens Seeing The Light

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Application

The Third and a Half Order lens was used almost exclusively on the Great Lakes, and naturally enough filled a void between the Third and Fourth order lenses. These lenses were used in coastal lights and for marking sounds. Of the twelve such lenses incorporated in US Lighthouses, seven were installed in Lights on the Great Lakes. These seven were Eagle Bluff, Detour Reef, St. Helena Island, Michigan Island, Huron Island, Grays Reef and Toledo Harbor.

Specifications
Lens height: 40" (estimate)
Focal length:  14.7" 
Weight of assembly:  1,000 lbs. (estimate)
Relative brightness (over sixth order): X3 (approx)
Useful range:  Up to 17 miles.
Number in US Lighthouses in 1884: 11
Number in US Lighthouses in 1922:  25
Oil consumption per hour 6 Ounces (estimate)

References

Annual Report of the Lighthouse Board, 1884
The United States Lighthouse Service, Dept of Commerce, 1915
Aids to Navigation Manual, US Coast Guard, 1945
Comparative Table of Lens Orders, Mabel A. Baiges, 1988
Letter from Helen & Al Gademsky, 07/02/01

Terry Pepper. This page updated 12/02/2007

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