| The nature of the white light spectrum produced
by a triangular glass prism is a function of the type of glass employed.
If the glass has a large overall index of refraction, the spectral lines
are at large angles to the incoming light beam. If the index of refraction
varies strongly with color, the angular spread or dispersion of the visible
spectrum between the red and violet edges will be large. The Polyprism
shows the variations in these two parameters by passing light through a
stack of triangular prisms made of different types of glass. The spectra
are displayed, one above another, on a nearby white wall.
The polyprism with ten glass segments at the left was made by Lerebours et Secretan of Paris, and is listed at 90 francs in the 1853 catalogue. The model with six layers of prisms was 50 francs. This device is at the United States Military Academy at West Point, New York.
The polyprisms at Allegheny and Union Colleges are by Duboscq of Paris (the form of attachment of the prism is shown in the Pellin/Duboscq catalogue ca. 1900), and those at Amherst and St. Mary's Colleges are also Duboscq, but probably of an earlier date. Amherst lost all of its apparatus in a disastrous fire in 1882, and the replacement apparatus was probably bought soon afterward.
Multiple liquid-filled prisms allow the same demonstrations to be done with various liquids.
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