This article from the May 14, 1965 issue of the Manned Spacecraft Center Roundup describes an "eye chart" experiment on an upcoming Gemini flight. Did this experiment end up happening in any manner?
Giant 'Eye Charts' To Check Visual Perception In Space
How well astronauts can see the Earth and objects on the Earth from an orbiting spacecraft will be determined scientifically in a project that will use areas in Australia and the southern part of Texas for the tests.
The scientific program of some future Gemini flight will include an experiment to shed light on this subject, which has been of interest to scientists since several astronauts in orbit reported seeing surprising detail on the Earth.
Main objective of the experiment will be to measure the cut-off point in recognizing objects on the ground as they are displayed in a progressively smaller fashion, much as an ordinary eye chart uses lettering of decreasing sizes.
Australia has agreed to provide a site in Western Australia which is well suited to Gemini flightpaths in the Southern Hemisphere, providing a sufficient number of sightings for the results to be statistically significant. A similar Northern Hemisphere area is being prepared 40 miles north of Laredo, Texas.
Flat plots of land in a semi-arid region are required for such an experiment so that cloud-cover interference is less likely. The terrain needs to be either barren of vegetation or easily cleared. The plots will range in size from a mile or more wide to several miles long.
Each plot, or strip of land, will be divided into a number of squares with some of the squares containing markings for use in the sighting experiments.
The shapes, or markings, presented for viewing by the astronauts will be of white material, such as sea shells, spread on the ground. The markings will have varying sizes and orientations.
The astronauts will be asked to report the number and orientation of the markings they can observe. The astronauts also will be repeatedly checked in flight with an on-board device to test their brightness discrimination, contrast threshold and visual acuity, or sharpness.
As the flight continues, the several tests will help determine how the visual capability of the astronauts is affected by prolonged weightlessness, breathing pure oxygen, breathing under a pressure of five pounds per square inch, and other in-flight stresses.
The experiment for testing visual perception in space will have as its principal investigator on behalf of NASA, S.Q Duntley, director of the Visibility Laboratory of the Scripps Institute of Oceanography, University of California, LaJolla.