Thursday, May 30, 2013

Who Made That Eye Chart?

Buying eyeglasses in the 1700s was tricky: 

you had to diagnose the problem yourself and then pick the correct lenses. Some vendors  helped customers along by scratching ages onto their spectacles, so that a 40-year-old would be steered to “40” lenses­ — the assumption being that everyone’s vision deteriorated at a similar rate as they grew older

But these shortcuts no longer sufficed in the mid-19th century, as doctors began to understand that patients needed bespoke lenses.
At a hospital in the Netherlands, Dr. Franciscus Donders devised a method for diagnosing vision problems: he would ask people to gaze at a chart on a distant wall and report what they could see. Apparently too busy to make the chart 
himself, he enlisted the help of a colleague, Herman Snellen.

Snellen first printed up a chart with dingbats — squares, circles,
 plus signs — of various sizes, but that proved to be a bad idea. 
“When you look at a symbol, how do you describe it?” says 
August Colenbrander, a scientist at the Smith-Kettlewell Eye
 Research Institute who has researched the history of the eye
 chart. (It was probably easy to cheat, too.) In the end, Snellen 
realized that letters would work best.

The chart spread all across Europe, an instant hit. “The first big 
order was from the British Army, in 1863 or so,” Colenbrander
 says. “Obviously the soldiers who fired their muskets had to be 
able to see.” Soon after that, printers everywhere copied it. A
 low-tech solution to a complex problem, the chart has remained
 popular because it was cheap and easy to use. But widespread 
reproduction and success are two different things. The EFPTOZ 
chart (pictured here) has practically become iconic, undermining
 its medical value, Colenbrander says. A recognizable eye chart is
 not a very useful one.

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