Interference Prism
This device is related to Newton's Rings, in which two glass surfaces, one flat and one convex, are pressed into close contact, forming a thin film with circular symmetry. In this case, the light is introduced into the system at a large angle of incidence by means of a prism that also forms the plane surface. This avoids the loss of signal by reflection from the upper side of the usual parallel-sided upper plate of glass. 

   The devices at the left and middle are at West Point, while the one at the right is at Allegheny. The two examples on stands were made by Duboscq of Paris. 

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