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!
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