Fresnel's and Lloyd's Mirrors
   Thomas Young's 1800 demonstration of interference used two pinholes as sources. To ensure that the signals from each were in phase, they were both illuminated by the same pinhole. The use of two real sources was considered unsatisfactory, and about 1816 Agustin Fresnel (1788-1827) used a pair of mirrors that produced two virtual sources from one real source. The mirrors are inclined at a slight angle to each other. and illuminated by a monochromatic light source with a beam of light at a small angle to each mirror. The two reflected beams, appearing to come from two closely-spaced virtual sources, interfere with each other and produce maxima and minima. 

   The apparatus is in the Jack Judson Collection at the Magic Lantern Museum in San Antonio, Texas, and appears to have been home-built.


   Lloyd's mirror is another device developed during the early part of the 19th century to demonstrate interference. Here, light from a monochromatic slit source reflects from a glass surface and a small angle, appearing to come from a virtual source. The reflected light interferes with the direct light from the source, forming interference fringes. Humphry Lloyd published an article about the demonstration in the Proceedings of the Royal Irish Academy of Science in 1837.

   This apparatus is part of a large projection lantern system made by E. Leybold's Nachfolger of Cologne. It is normally placed horizontally and attached to the front of the lantern. The slit and reflecting surface can be clearly seen; the beam-blocker perpendicular to the mirrored surface is used to prevent unwanted direct light from flooding the projection screen. 

   This apparatus is in the Jack Judson Collection at the Magic Lantern Castle Museum in San Antonio, Texas. 

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