Page:Popular Science Monthly Volume 91.djvu/28

 Tests Which Scientists Use to Show Why We Can

��The eye is a kind of pho- tographic camera. You don't always want a bright light when you photograph, and neither does the eye when it sees. A camera has a dia- phragm which can be opened more or less to regulate the amount of light admitted; the eye has an iris which does the same automatically. But the eye can be fooled. In the picture the observer is look- ing through a lens at an object. When the object is moved farther away and the lens removed the iris opens to see better, and yet the intensity of the light has not changed and the widen- ing of the iris is unnecessary

��Photos by Jacques Boye

����The "olfactometer" which de- termines the density of vapor at which the first perception of smell is possible. By its use we learn that there are many odors which we do not perceive

��At left: An interesting test to prove to the human brain that the muscles can lift a greater load than seems possible. The load is increased very gradually

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