aberroscope

aberroscope 

Instrument for observing aberration. Such an instrument was designed by Tscherning to measure his own spherical aberration. It consists of a planoconvex lens with a grid made up of squares ruled on its plane surface.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann
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He uses a device called an aberroscope, which projects a grid onto the retina.