Eyeglasses for Dogs.

Apparently, many dogs are nearsighted – hence the need for prescription eyeglasses. But how to tell when you’ve got the prescription right?

"Since a dog can’t be asked to read letters on a chart, a dry, odorless bone is dangled in front of him. If he shows any interest, the optician knows that the test lenses are right"

From April 1939 issue of Popular Science: Link – Thanks Charlie!