Blink comparator, made welknown by Pluto discoverer Clyde Tombaugh in 1930, originated from the early 1900s.
Carl Purfrich invented the stereo-comparator in 1900 and in 1904 he added a monocular microscope attachment to the comparator due to his lost of one eye sight, the blink comparator was born when he made the line of sight alternate between plates.
Source:
Title: Notes on the History of the Blink Comparator
Authors: Drummeter, L. F., Jr.
Journal: Bulletin of the American Astronomical Society, Vol. 23,
p.1347
Bibliographic Code: 1991BAAS...23.1347D