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Ask Marilyn: What Snellen Was Thinking?
Mitchell Drucker of Tampa, Florida, writes:
Marilyn: I have enjoyed your column for many years. I especially liked your explanation about 20/20 visual acuity. (October 7, 2012) The background story behind Snellen's fraction is interesting. Presumably he was looking at the second star in the handle of Ursa Major and remarked to a friend that it was a double star. The friend could not visually appreciate the binary system. Snellen plotted the angle that the two stars would have to subtend on the retina to be resolved as two stars. This is supposedly the basis of 20/20 (6/6) acuity; being able to resolve Mizar and Alcor as two separate entities.
My son's science project in sixth grade lent credence to the story as 100% of the 30 people he tested with 20/20 acuity saw the paired stars as two bodies, but no one with 20/25 or worse acuity could resolve them. He has since graduated from Stanford and is pursuing a PhD at Emory, but this project may have been his best research to date.
What a great story! Thank you. Readers, Dr. Drucker is an Associate Professor of Ophthalmology and Neurology at the University of Southern Florida.