Lenses
   Both of these Duboscq lenses are at the United States Military Academy at West Point, New York.

   At the right is a plano-convex lens with a 7 ¾ inch clear aperture.

   The lens at the left is a hollow plano-convex lens. It can be filled with liquids of different indices of refraction, allowing its focal length to be changed. The focal length can then be compared with that predicted from the Lens-Maker's Equation.
The converging lens at the left at Washington and Jefferson College in Washington, Pennsylvania, is by John Browning of London. 

   At the right is a Duboscq diverging lens at Allegheny College in Meadville, Pennsylvania. It is 40 cm in overall height, and has a 9.7 cm clear aperture.
   The three lenses at the left are in regular use in lecture demonstrations at the University of Cincinnati. The sheet brass outlines show that the lenses are double-convex (converging), plano-convex (converging) and double-concave (diverging). 

   The set is marked Jules Duboscq/Ph PELLIN/Paris. Jules Duboscq (1817-1886) was succeed in business by Phillippe Pellin.
   Two more Duboscq converging lenses: At the left is one from Allegheny whose focal length I measured in 1978 to be about 45 cm. This is a big piece of apparatus with an overall height of 54 cm. 

   The converging lens at the right is at the United States Military Academy.
 
   This lens is at Denison University in Granville, Ohio. I have seen circular plane mirrors set into similar frames, making me wonder if a Denison student or faculty member in the 1890-1910 era, removed the mirror and replaced it with the lens, while also adapting it for use in an optical bench.
 
   The large burning glass on the left is on display at the University Museum of the University of Mississippi in Oxford. It was bought from Lerebours et Secretan of Paris in the second half of the 1850s by Prof. F.A.P. Barnard.

   The glass, which cost 1800 francs (about $350) is twenty three inches in diameter. The stepped or "echelon" lens is due to Augustin Jean Fresnel (1788-1827) in the last few years of his life and was developed for use in lighthouses. One surface of the lens is flay, and the other is cut into a series of curved concentric rings that are portions of a spherical surface. The lens is thus made much lighter than a solid lens. In the middle of the 19th century the apparatus was used to melt refractory metals held at the focal point of the lens.
   This is another Duboscq converging lens, similar to the ones abbove from Allegheny and West Point. It has a clear diameter of about 19 cm. 

   This example is in use in lecture demonstrations at Cornell University.   
 
 

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