This paper summarizes the history of research, particularly a pivotal work published in 1966, on visual accommodation relevant to perceived size of distant objects. In 1947, Alexander Williams proposed using a periscope as the basis for a flying display simulator in anticipation of augmented contact and sensor-relayed contact displays. However the unity image had to be magnified in order to achieve normal landing performance. This successful intervention, first published in 1966 in Human Factors, implicated oculomotor mechanisms and higher perceptual functions and became the observational basis for a series of investigative hypotheses. Observers registered the perceived size of the collimated image of a "moon" by adjusting a disk of light while alternatively providing optometric measurements of accommodative distance. Studies found high correlations between focal distances and perceived moon sizes. The simulated moon provided a superior vehicle for revealing the relationship between focal distance and perceived size and the factors affecting both. The operational display design implications and the possibility of a partial explanation for the moon illusion provided the motivation for an important doctoral research project involving eight factors that affect both focal distance and perceived size. The investigation reaffirmed that virtual images, as found in head-up and head-mounted displays, do not consistently draw focus to optical infinity. These findings have important implications for aviation display designers.
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