Legibility and Readability

In Letters of Credit: A View of Type Design, Walter Tracy explains the different between legibility and readability:

Legibility, then, refers to perception, and the measure of it is the speed at which a character can be recognised; if the reader hesitates at it the character is badly designed. Readability refers to comprehension, and the measure of that is the length of time that a reader can give to a stretch of text without strain.