Saturday, May 24, 2008

The One Weak Eye


A Simple Experiment by Which It May Be Discovered.

"Yes," said the doctor in a moment of unprofessional confidence, "the makers of optical instruments are turning out some wonderful appliances nowadays for discovering imperfections of vision, but I'll tell you of a plan for testing the respective strength of your eyes that is as simple as it is trustworthy. All you need is a stereoscope and a photograph. That arrangement in which the picture holder slides up and down a flat frame, trombone fashion, is the best sort of stereoscope for the purpose, although any will do, and the photograph that will give the best results is a cabinet size view of some locality with people in it. The modus operandi is simplicity itself.

"Put the photograph in the holder and focus it just enough so that you can see the faces clearly. Then close the left eye and look at the picture intently with your right eye while you count 80 slowly. Now close the right eye and look at the picture with the left eye for the same space of time. Then open both eyes and look at the picture without changing the focus. Something queer will happen. The figures on the one side of the picture will seem to move across the view and group themselves with those on the other side, and — this is the point of the experiment — the figures will always move away from the weak eye. Moreover, they will move with a very precise relation of speed to the weakness of vision. If the left eye, for example, is quite weak, the figures will move very quickly across the plane of sight to the right side, while if there is but a slight defect the movement will be gradual, and so on.

"A queer thing about this experiment is that, simple as it seems, it will bring out defects of vision that have never been suspected, and another queer thing is that it will demonstrate the cases in which both eyes are of equal power to be surprisingly exceptional. I have tried it in a score of mixed gatherings and never yet without having the experimenter observe some movement of the figures. There was one old lady, I remember, up at Port Jefferson, who persisted in saying that she saw precisely with both eyes as she did with one eye, and well she might, for when I examined her eyes closely I found she was stone blind on the left side and didn't know it." — New York Sun.

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