Page:The fairy tales of science.djvu/135

Rh If we look at a candle placed at a distance of about ten feet, we see it distinctly as one object, because our eyes are so adjusted that the image of the candle is projected on similar parts of each retina. But if we now hold up a finger about ten inches from the eyes, and look steadily at it, the candle will be seen on both sides of the finger. The eyes are now adjusted to the finger, and the image of the candle no longer falls on the same parts of the two retinæ. Again, if the eyes be directed to the light, the finger will be seen double, because the optic axes are now adjusted to perceive objects at a distance of ten feet. Similar effects may be produced by pressing one eyeball with the finger so as to displace its optical axis, or by getting intoxicated, an experiment which we trust our readers will never perform.

We make use of our two eyes as a pair of compasses to measure distances, for we involuntarily associate the idea of smallness with the convergence of the visual axis, and that of vastness with its divergence. We feel that an object is near or remote, small or large, by opening and shutting our magic compasses, the legs of which are imaginary lines passing through the eyeballs. A person suddenly deprived of one eye estimates the distance of objects with the greatest difficulty; but after some time, experience teaches the one eye to measure distance by the change of focus alone. Let the reader close one eye, and try to snuff a