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 Binocular Vision and Experiment

Although both eyes are kept open to line up the pencil and the distant object, only one eye, the dominant eye, is used to determine the alignment. When this eye is closed, the pencil will appear to 'jump' sideways. It is closure of the dominant eye which produces the jump because, according to the eye now remaining open, the pencil and distant object are not in line. If the student is slow to line up the two objects he or she may be aware of a double image of the near object. Nevertheless, the student is still likely to select the image from the dominant eye.

Keep both eyes open and hold a pen or pencil upright at arm's length.
Quickly move the pencil to come exactly in line with a more distant vertical object, such as a window frame or lamp-post.
Close and open the left eye.
Close and open the right eye.
Note any change in the apparent position of the pencil and whether it was the closure of the left or right eye which produced it.
Which eye did you use in lining up the two objects?
Place a glass marble on a non-slippery surface, e.g. on the page of an open notebook.
Cross the first and second fingers of one hand and press on the marble with the tips of these fingers.
Close your eyes and roll the marble firmly, forwards and backwards, side to side and then with a circular motion for about 30 seconds.
Notice any unusual impressions you receive from the fingers.

Source: Compact Porro Binoculars,    Monocular Telescope

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