WHO put E on top of the eye chart? And WHY?

Hermann Snellen, a Dutch professor of ophthalmology, put the E on top of eye chart in 1862. Although his very fisrt chart was headed by an A, Snellen quickly composed another chart with E on top.

Snellen succeeded Dr Frans Cornelis Donders as Director of the Netherlands Hospital for Eye Patient. Donders was then the world's foremost authority on geometric optics. Snellen was trying to standadise a test to diagnose visual acuity, to measure how small an image an eye can accept while still detecting the detail of that image.

Donder's complicated formulas were based on three parallel lines; of all the letters of the alphabet, The capital E most closely resembled the lines that Donders had studied so intensively. Because Donders had earlier determined how the eye perceives the E, Snellen based much of this mathematical work on the fifth letter.

The three horizontal limbs of the E are separated by an equal amount of white space. In Snellen's original chart, there was a one-to-one ratio between the height and width of the letters, and the gaps and bars were all the same length (in  some modern eye charts, the middle bar is shorter).

Lounne Gould of Cambridge Instruments says that the E, unlike more open letter like L or U, forces the observer to distinguish between white and blanck, an important constituent of good vision. Without this ability, Es begin to look like Bs, Fs, Ps or many other letters.

Of course, Snellen couldn't make an eye chart full od only Es, or else all his patients would have 20-10 vision. But Snellen realised that it was important to use the same letters many times on the eye charts, to ensure that the failure of observer to identify a letter was based on a visual problem rather than the relative difficulty of a set of letters. Ian Bailey, Professor of Optometry and Director of the Low Vision Clinic at the University of California at Berkeley, says that it isn't so important whether an eye chart uses the easiest or most difficult letters. Most eye charts incorporate only ten different letters, ones that have the smaller range of difficulty.

Today, many eye charts do not start with an E - and there is no technical reason why they have to - but most still do. Dr Stephen C Miller of the American Optimetric Association suggest that the desire of optical companies to have a standardised approach to the production of eye charts probably account for the preponderance of E charts.

And we're happy about it. It's a  nice feeling to know that even if our vision is failing us miserably, we'll always get the top row right.