Seven Mirror Apparatus
   The problem is to reverse Newton's demonstration of the spectrum, in which a beam of sunlight coming through a small hole in a window blind is spread out into a spectrum after passing through a triangular prism. Newton assumed that there were seven colors: red, orange, yellow, green, blue, indigo and violet. Most of us would omit indigo, but Newton felt that seven was a sacred number, so there had to be seven colors. 

   The straightforward solution is to pick off samples of each of the colors with a small mirror, and reflect the light back to a common point.

                              Union College                                                                       Smithsonian Institution

Neither of these pieces of apparatus is marked, although both of them may be French. Other methods of spectrum recombination are Newton's Color Wheel and the Oscillating Prism.


The Seven-Mirror Device at the left is marked Max Kohl of Chemnitz, Germany and is in the collection of Hobart and William Smith Colleges of Geneva, New York.

   It dates from the later 1920s, and the Kohl catalogue of that era calls it an "Apparatus with 7 Mirrors, for re-combining the light split up into the spectrum colours; consisting of 7 plane mirrors 55 mm diameter, in mounts, movable in all directions, on adjustable stand."

REF: Thomas B. Greenslade, Jr., "Spectrum Recombination", Phys. Teach., 22, 105-108 (1984)

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