[visionlist] Roman acuity test?

[Denis Pelli]

Reading various posting found through Google, I think the following is
true:

"The Roman legions required each candidate for Centurion Guard to
resolve a pair of stars, Alcor and Mizar in Ursa Major, 12 min arc
apart."

However, I would like to publish this in an article. Can anyone specify
a reference that I can cite for this fact?

Denis Pelli
Professor of Psychology and Neural Science
New York University
http://psych.nyu.edu/pelli/

p.s.
Were Centurion Guards lookouts?