Lamps and Lenses: How Old Mackinac Point Shed its Light

What puts the light in a lighthouse? Until 1913, a single lamp provided the light for the beacon at the Old Mackinac Point Lighthouse.

This lamp is similar to the one used in the first 20 years of the lighthouse’s existence. It was eventually replaced with a gas lamp and then an electric light.

This diagram shows how even a small amount of light from the lamp could be magnified using a number of prisms in a Fresnel lens. (Click to enlarge)

Like most other lighthouses in the United States, Old Mackinac Point used a kerosene lantern not unlike those still found in many homes today. Keeper George Marshall’s duties included regularly fueling the lamp, as it burned over 5 ounces of kerosene per hour. A red glass lamp chimney originally produced a red light, helping sailors identify Old Mackinac Point. Both the kerosene lamp and red light were replaced in 1913, when the U.S. Lighthouse Service installed an incandescent oil vapor mechanism, which produces a white light similar to a modern camping lantern.

While the kerosene lamp itself shone brightly, a fourth-order Fresnel lens amplified the Old Mackinac Point light until it was visible to a distance of 16 miles. The lens contained a series of stacked magnifying glasses around its center, with each glass focusing the light from the one beneath it. Prisms ringed the lens above and below the magnifying glasses. These prisms caught and bent more light, focusing it into the central beam created by the magnifiers. The entire lens rotated around the lamp, creating a flashing signature (one flash every 10 seconds) unique to Old Mackinac Point.