From Ask Yahoo!:
Dear Yahoo!: Why is the letter “E” on top of all eye charts?
The standard eye chart is called the Snellen Chart, after the Dutch ophthalmologist Herman Snellen (1834-1908). Snellen developed the chart in 1862; it measures visual acuity, or the ability to see from a fixed distance.
Why the big “E?” That’s how Snellen designed the original, and having a “standard letter” on top helps to determine the chart’s size and the distance it should be from the patient. However, we found several variations of the Snellen chart: the letter “A” on top, “K V D” on top, as well as the letter “F“.
The size of the letters relative to each other is very important. Snellen letters, also known as optotypes, grow progressively smaller according to a constant ratio. The eighth row of letters is usually the 20/20 line. Someone with 20/20 vision has “normal” vision (though 61% of Americans wear corrective lenses), while someone with 20/40 vision can only see at twenty feet what a normal person can see from forty feet.
Some charts use a series of “Es” facing different directions; the objective is to identify the directions of the letters. This is a popular test for children or people who can’t read.