A Hinman Collator is a pre-digital machine for comparing two different copies of a printed page in order to detect any differences between them. Lights and mirrors allow you to see the two images superimposed one on top of the other, at which point any small differences between them will hit you in the eye.

The Folger Library blog, The Collation, has a piece by Andrew R. Walkling. This includes an animated clip showing dancing before your eyes the difference between two versions of a line of type.

The device was invented by Charlton Hinman  (1911-1971) in the late forties and drew upon his wartime work on aerial photography. Its main market was among bibliographic research institutions, but it is alleged that the CIA did buy one for more practical purposes. Hinman was the editor of the Shakespeare Quarto Facsimiles and The Norton Facsimile: The First Folio of Shakespeare. From 1960 till 1976 he was professor of English at the University of Kansas.