Moreover, as shown below, it is needed by children able to pass satisfactorily the snellen test.
"Civics and Health"
William H. Allen
If the regular snellen card is used containing letters of different size, place in the denominator the number of the lowest line each eye and both eyes together can read easily, and in the numerator the number of feet from card to eye.
"Civics and Health"
William H. Allen