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SECOND. A measure equal to one sixtieth part of a minute. Vide Measure.

A Law Dictionary, Adapted to the Constitution and Laws of the United States. By John Bouvier. Published 1856.
References in periodicals archive ?
Uretmen et al found 480 seconds of arc or better was present in 21.8% of patients and 1.5% had 60 seconds of arc or better stereoacuity.4 Berk et al.
It was also claimed that the optometrist did not undertake retinoscopy with a cycloplegic agent in circumstances where the examination indicated a visual acuity of 3/9, stereo acuity was 200 seconds of arc, there was limited cooperation from the child patient, and the reason for the examination was she had been squinting and closing her eyes when watching television.
Thus .308" is .308 / (1.047) = .294 Minutes of Arc at 100 yards or .294x60 = 17.65 Seconds of Arc, and .294 / 6 = .049x60 = 2.49 Seconds of Arc at 600 yards.