Look Away to See Better?

fatigue of the eye

One of my favorite things about reading old issues of The Chatham Record is coming across articles that explain odd phenomena. This one from November 5, 1896 tells us why stars seem to dim when we look right at them.

 

Fatigue of the Eye

A star appears more distinct if one looks at a point near it than it does when looking at the star itself. This is due to what is called “fatigue” of the retina. When looking at an object we naturally focus that object on the most sensitive part of the retina and keep the image permanently there. Now, in the case of a star, the image is a microscopic point and covers only an infinitesimal portion of the retina, and the great strain on this portion produces immediate “fatigue.” If we look at a point near the star then as they eye moves, the image travels over the retina and successive portions of it are called into play so quickly that “fatigue” is not experienced. It does not do to look at another star near it, for then the eye is kept fixed and “fatigue” at once ensues. To most eyes the Pleiades appear far more distinct when we fix our eyes on a blank space of the heaven near them than when we look at them directly.

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