[visionlist] Re: Roman acuity test? - UPDATE

Denis Pelli denis.pelli at nyu.edu
Sun Aug 5 08:32:22 GMT 2007

hi all,

here's an update.

so far, the only authoritative source for the Alcor/Mizar acuity test  
is this link to Encyclopedia Britannica provided by Ronald C Blue:

Based on that, reorganizing the information for a vision audience, I  
tentatively plan to say this in my paper:

The ability to see the dim star Alcor (from Arabic “faint one”), in  
the middle of the handle of the Big Dipper (Ursa Major), with the  
unaided eye may have been regarded by the Arabs (and others) as a  
test of good vision (Encyclopedia Britannica). Alcor (magnitude 4.01)  
is 20% as intense as Mizar (Arabic: “Veil,” or “Cloak”, magnitude  
2.27), which is only 12 min arc away, forming a visual double.

Alcor. (2007). In Encyclopædia Britannica. Retrieved August 5, 2007,  
from Encyclopædia Britannica Online: http://www.britannica.com/eb/ 

We're contacting two experts who may yet provide documentation of the  
Roman use of Alcor and Mizar to test acuity.
Andy Herbert (Psychology, RIT) knows an expert on the Roman army.
Jay Enoch (Optometry, UC Berkeley) notes that:
> "At the Cogan Ophthalmic History Society (Ophthalmology) meeting in  
> San Francisco this past April or March, Dr. George Bohigian,  
> Ophthalmologist from Saint Louis, gave a talk on this ancient pair  
> of stars being used as a test of visual acuity.  He might know the  
> answer to your question.  He had been a student of mine.  Jay Enoch"

Many people noted that a Centurion is a leader of one hundred men,  
and could not be a "Guard".

Larry Maloney <laurence.maloney at nyu.edu> and Jonathan Victor  
mentioned another ancient celestial vision test:
> The number of Pleiades visible to the naked eye is variable,  
> depending on the eye
> and viewing conditions.  But folks who can see seven should  
> probably also be able to see
> an 8th. The 7th and 8th Pleaides in order of descending visibility  
> are very close.
> http://www.naic.edu/~gibson/pleiades/pleiades_see.html
> I've never seen more than six but the number potentially visible  
> with the unaided eye
> is closer to 20. It would be interesting to have a translation of  
> number seen into a standard
> measure of visual sensititivity ....

Koorosh Mirpout <mirpour at gmail.com> and Ol Braddick  
<Oliver.Braddick at psy.ox.ac.uk> mentioned the thoughtful note by  
Maurice Wooldridge in the BMJ, which gently contradicts the brash  
assertion by Moore's astronomy book that resolving the 12' separation  
of Alcor and Mizar would be too easy to serve as a useful vision  
test. Wooldridge, unlike Moore, took the trouble to look. The unequal  
5:1 brightness of the two stars makes this unlike the usual two-point  
acuity task.
Moore P. Astronomers stars. London: Routledge and Kegan Paul, 1987:22-3.

Ian Bailey (Optometry, UC Berkeley) adds,
> I had not heard the Roman's stars story. Early papers used to say  
> that Thomas Hooke-  a British astronomer I think - said something  
> like ---a person with good average vision could just resolve 2  
> stars when they were separated by 1 minarc.

thanks to all.



Denis Pelli
Professor of Psychology and Neural Science
New York University

On Aug 4, 2007, at 10:42 PM, Ronald C Blue wrote:

> http://www.britannica.com/eb/article-9000495/Alcor
> -----Original Message-----
> From: "Denis Pelli" <denis.pelli at nyu.edu>
> To: "CVNET" <cvnet at mail.ewind.com>; visionlist at visionscience.com
> Sent: 8/4/07 2:33 PM
> Subject: CVNet - Roman acuity test?
> 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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