| A prism can be used
to split a narrow beam of white light into a spectrum. Conversely, it can
also be used to reverse the process. One way to do this is to produce a
spectrum with a prism, and then blur it by rocking the prism.
In both of these examples, the prism is rocked by turning the crank. The device at the left is in the National Museum of American History at the Smithsonian Institution. On the right is an oscillating prism made by C. Gerhardt of Bonn and in the collection at Dartmouth College.
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