138 Memory as an Aid to Vision
When the two eyes of a patient are different, it has been found that the difference can be exactly measured by the length of time a black period can be remembered, while looking at the Snellen test card, with both eyes open, and with the better eye closed. A patient with normal vision in the right eye and half-normal vision in the left could, when looking at the test card with both eyes open, remember a period for twenty seconds continuously; but with the better eye closed, it could be remembered only ten seconds. A patient with half-normal vision in the right eye and one-quarter normal in the left could remember a period twelve seconds with both eyes open, and only six seconds with the better eye closed. A third patient, with normal sight in the right eye and vision of one-tenth in the left, could remember a period twenty seconds with both eyes open, and only two seconds when the better eye was closed. In other words, if the right eye is better than the left, the memory is better when the right eye is open than when only the left eye is open, the difference being in exact proportion to the difference in the vision of the two eyes.
In the treatment of functional eye troubles this rela tionship between relaxation and memory is of great prac tical importance. The sensations of the eye and of the mind supply very little information as to the strain to which both are being subjected, those who strain most often suffering the least discomfort; but by means of his ability to remember black the patient can always know whether he is straining or not, and is able, there fore, to avoid the conditions that produce strain. What ever method of improving his sight the patient is using, he is advised to carry with him constantly the mem ory of a small area of black, such as a period, so that