Fig. 72.--By aid of this simple figure, the Poggendorf and barber-pole illusions can be seen to be instances of the Müller-Lyer illusion, Try to bisect the horizontal line in this figure. The oblique line at the right tends to displace the right-hand end of the horizontal to the right, while the oblique at the left tends to displace the left-hand end of the horizontal also to the right. Similar displacements account for the Poggendorf and barber-pole illusions.
Fig. 73.--The Zoellner illusion. The long lines are really parallel. The illusion is increased by holding the figure so that these main lines shall be neither vertical nor horizontal. It is more difficult to "deceive the eye" in regard to the direction of vertical and horizontal lines, than in regard to the direction of oblique lines. This illusion must be related in some way to the Müller-Lyer and Poggendorf illusions, since the elements employed in constructing the three figures are so much the same.
If you treat this figure according to the directions given for [Fig. 67], and sight along the obliques, you get an illusion of perspective.
It is called the "size-weight illusion", and may be said to be based on the old catch, "Which is heavier, a pound of lead or a pound of feathers?" Of course, we shrewdly answer, a pound's [{460}] a pound. But lift them and notice how they feel! The pound of lead feels very much heavier. To reduce this illusion to a laboratory experiment, you take two round wooden pill-boxes, one several times as large as the other, and load them so that they both weigh the same; then ask some one to lift them and tell which is the heavier. He will have no doubt at all that the smaller box is the heavier; it may seem two or three times as heavy. Young children, however, get the opposite illusion, assimilating the weight to the visual appearance; but older persons switch over to the contrast effect, and perceive in opposition to the visual appearance. What seems to happen in the older person is a motor adjustment for the apparent weights, as indicated by their visual appearance, with the result that the weight of larger size is lifted more strongly than the weight of smaller size; so that the big one comes up easily and seems light, the little one slowly and seems heavy.
EXERCISES
1. Outline the chapter.
2. Show that the law of combination accounts both for many correct perceptions, and for many illusions.
3. Through which of the senses are spatial facts best perceived?
4. "At first, the baby very likely perceives a ball simply as something for him to handle and throw; but, through the medium of blocked response, he comes to perceive it more objectively, i.e., as an object related to other objects, and not simply related to himself." Explain and illustrate this statement.
5. Give an example from the field of auditory perceptions where "isolation" is very much in evidence.
6. Can you see any law analogous to Weber's law in the field of financial profit and loss? Does a dollar gained or lost seem the same amount, without regard to the total amount possessed?
7. Trial and error perception. Go about the room with closed eyes, and identify objects by touching them with the hands. Notice whether your first impression gives place to corrected impressions.
8. Perception of form by "active" and "passive" touch. With the eyes closed, try to distinguish objects of different shapes (a) by letting them simply rest upon the skin, and (b) by handling them. What senses coöperate in furnishing data for "active touch"?
9. Binocular parallax, or the differing views of the same solid object obtained by the two eyes. Hold a small, three-dimensional object a foot in front of the face, and notice carefully the view of it obtained by each eye separately. A pencil, pointing towards the face, gives very different views. What becomes of the two monocular views when both eyes are open at once?
10. Binocular compared with monocular perception of "depth" or distance away. Take a pencil in each hand, and bring the points together a foot in front of the face, while only one eye is open. When the points seem to be nearly touching, open the other eye, and see whether the two points still seem to be close together. Repeat.