by eye en · ADP
Meanings
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By estimation, based on a visual inspection, without accurate measurement.
Thus given a straight line it is very easy to construct a smaller or a greater than it by eye, not just because this is a broad sort of distinction, but because it also involves only one comparison.
1984, Andrew Barker, Greek Musical Writings: Volume 2, Harmonic and Acoustic Theory, page 277:
Coordinates
under the spotlight · under the magnifying glass · under the loupe · under the microscope