You will have a clearer understanding of the sense-perceptive processes and a more vital realization of the practical significance of these facts when you consider how they affect your knowledge of material things and your conception of the external world. This subject possesses two distinct aspects. One aspect has to do with the inability of the sense-organs to record the facts of the outer world with perfect precision. These organs are the result of untold ages of evolution, and, generally speaking, have become wonderfully efficient, but they display surprising inaccuracies. These inaccuracies are called Sensory Illusions. [Sidenote: _Practical Aspects of Perception Process_] The other aspect of the Sense-Perceptive Process has to do with the mental interpretation of environment. Both these aspects are distinctly practical. You should know something of the weaknesses and deficiencies of the sense-perceptive organs, because all your efforts at influencing other men are directed at their organs of sense. You should understand the relationship between your mind and your environment, since they are the two principal factors in your working life. CHAPTER III SENSORY ILLUSIONS AND SUGGESTIONS FOR THEIR USE [Sidenote: _Unreliability of Sense-Organs_] Figure 1 shows two lines of equal length, yet the vertical line will to most persons seem longer than the horizontal one. [Illustration: FIG. 1.] In Figure 2 the lines A and B are of the same length, yet the lower seems much longer. [Illustration: FIG. 2.] Those things look smallest over which the eye moves with least resistance. In Figure 3, the distance from A to B looks longer than the distance from B to C because of the time we involuntarily take to notice each dot, yet the distances are equal. [Illustration: FIG. 3.] [Sidenote: _Being and Seeming_] For the same reason, the hatchet line (A-B) appears longer than the unbroken line (C-D) in Figure 4, and the lines E and F appear longer than the space (G) between them, although all are of equal length. [Illustration: FIG. 4.] Filled spaces look larger than empty ones because the eye unconsciously stops to look over the different parts of the filled area, and we base our estimate upon the extent of the eye movements necessary to take in the whole field. Thus the filled square in Figure 5 looks larger than the empty one, though they are of equal size. [Illustration: FIG. 5.] White objects appear much larger than black ones. A white square looks larger than a black one. It is said that cattle buyers who are sometimes compelled to guess at the weight of animals have learned to discount their estimate on white animals and increase it on black ones to make allowances for the optical illusion. [Illustration: THIS MAN AND THIS BOY ARE OF EQUAL HEIGHT, BUT ASSOCIATION OF IDEAS MAKES THE MAN LOOK MUCH THE LARGER] [Sidenote: _Use of Illusions in Business_]