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Abstract: The effects of dial scale mark length, thickness, spacing and observing distance onreading speed and accuracy were studied under laboratory conditions. Preliminary ex-periments with simplified scales revealed that length and thickness are not as important asseparation of scale marks. These findings were verified by using simulated instrumentials as testing objects. Further experiments indicated that there exist lower limits forlength and thickness of scale mark, that the equal visual angle made by scale markdimensions and separation gives equal legibility, no matter what the observing distanceis, and that the legibility is a function of the visual angle made by the separation betweencale marks. Under our experiment conditions, this relation can be expressed by thequations, RT=1.41/α+91.2 and E=0.17/α~(1.71), where RT denotes reading reaction time,E, per cent reading errors, and α, visual angle of separation between scale marks inegree.

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