Snellen Vision Testing Cards Set for Railway, Marine and School Exams, Early 20th C.

Found in their original envelop, with Millar and Wel_ (?) Opticians on, Boston stickeron the face and instruction card still inside, this appears to be a complete, or nearly so, set of early 20th c. Snellen vision testing cards. Nineteen cards total, plus instruction card, with three of the cards showing railway signals to be read at a distance of 20 feet--as these were commonly used for testing the vision of train conductors/railway workers. 

Included: directions for use; 3 rail signal cards + one lettered card to be read at a distance of 20 feet;  three 15 foot cards; three 30 foot cards, three 40 foot cards; three 50 foot cards; three 70 foot cards. Just cool, and I was thinking at the very least these would be fun to give to the kids for playing "eye doctor"!