[visionlist] Roman acuity test?

Denis Pelli denis.pelli at nyu.edu
Sun Aug 5 00:32:12 GMT 2007

dear gislin

in fact, distinguishing these stars, 12' apart, is a demanding  
criterion. they are not very bright. i suspect that they cannot be  
seen foveally (rod-free zone). this task may be contrast-limited.   
the background luminance of the night time sky is much higher for  
most of us than it was in Roman times.

i am trying to find out exactly what the test consisted of. i don't  
yet know how the question was posed to the observer. one might ask  
which star in the big dipper is double. or one might ask the  
orientation of the double star. it's not clear to me how one would  
reject a memorized answer.



Denis Pelli
Professor of Psychology and Neural Science
New York University

On Aug 4, 2007, at 7:29 PM, Gislin Dagnelie wrote:

> Denis,
> Whether it is true or not, I think most of us would not be very  
> impressed with a two-point resolution of 12 arcmin.
> Unless, of course, the 12 arcmin is the separation of Alcor and  
> Mizar in 2007, and their separation has increased over the past two  
> millennia.
> Constellations are known to change shape through the ages as our  
> terrestrial vantage point in space shifts due to parallax.
> You may want to check with someone in the Astronomy department to  
> figure out what the real acuity measure was.
> Gislin
> P.S.  A Roman centurion commanded a group of 100 men, so he would  
> be equivalent to a captain or so.  The term "centurion guard" does  
> not ring a bell with me in a Roman context, but the term is used in  
> World of Warcraft... Maybe that's where they use this acuity test?
>>>> Denis Pelli <denis.pelli at nyu.edu> 08/04/07 2:33 PM >>>
> 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?
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