This inflight vision tester was supplied by researchers as the main equipment for the S-8, D-13, and M-9 tests of the four Gemini astronauts on the Gemini V and VII missions. Based on a variety of visual cues supplied to test visual acuity and otolithic functions, astronauts recorded their responses with cameras, audio recorders, and on yellow cards inserted into the device. Researchers wanted to test the ability to see a variety of shapes and patterns while in orbit, and also determine to what degree the otolithic membrane (which provides a sense of being upright) in the inner ear was disrupted in weightlessness. These longer duration Gemini missions provided data that would inform doctors on Earth about the effects of prolonged weightlessness on different bodily functions.