So it falls into errors of judgment through the trap which is set by its own manner of working.

Nowhere is the matter better illustrated than in the sphere of vision.

The number of illusions of vision is remarkable.

We are constantly taking shapes and forms for something slightly different from what, by measurement, we actually find them to be.

And psychologists are attempting--with rather poor success so far--to find some general principles of the mechanism of vision which will account for the great variety of its illusions.
Among these principles one is known as Contrast.