The official eye chart was created by Herman Snellen, and it included symbols such as squares, circles and plus signs. After realizing the challenges of using these symbols, like the difficulty in describing each one, letters were put onto the chart.
In the 1700s, people diagnosed their eye problems independently, and then chose their glasses themselves. Some vendors put a number on the lenses. This was done assuming that vision followed the same pattern for everyone, meaning each 40 year old would require the same glasses. Doctors began to understand that a better method was needed to choose glasses. So Dr. Franciscus Donders of the Netherlands began asking patients to look at a chart on the wall and describe what they saw. He asked Herman Snellen to create the chart.
After Snellen made the lettered chart, it spread quickly across Europe, with the first big order going to the British army around 1863. Printers began copying the chart shortly thereafter.
On the original chart Snellen created, the progression of letter sizes is irregular. In 1868, Dr. John Green of St. Louis suggested a uniform progression of size for each line. He also suggested a non-serif font and proportional spacing. This sequence was not generally accepted until a century after his proposals.