degrees, when B F is really in the same plane as A B and B C, and therefore the angle AB F is part of the angle АВС?

Examine Fig. 3 in a similar way.

of lines in each figure?

E

Fig. 3

Describe the lengths

Is it more or less easy to see the figure as a cube when drawn in perspective as in Fig. 3, than when drawn with equal edges, as in Fig. 2? Why?

glass

Hold a piece of between your eye and a cubical aquarium with glass faces and black metal edges, and mark off on the glass in your hand the lines which lie between your eye and the edges of the aquarium. In what respects does

the picture you have drawn on the glass resemble Fig. 3? In what ways has past experience assisted you in interpreting the twelve lines in one plane in Fig. 3 in such a way as to give you a perception of a cubical block?

If in Fig. 2 the face ABCD seems nearer to you than the face E F G H, reconstruct the perception so that A B C D is at the farther end of the cube and E F G H at this end.

Similarly reconstruct the perception of Fig. 3.

Observe Fig. 2 and interchange A B C D and E F G H as rapidly as you can. Then observe the figure without any effort at reconstruction. Do you find that the shifting process continues and occurs at regular intervals?

Close the book. Make up your mind that when you open the book and look at Fig. 2 you will see E F G H as the face nearest to you. Open the book and look at Fig. 2. Does the observed figure conform to your preconceived perception? Give examples where desire and expectancy have

caused persons to think they have seen what they really have

not seen.

Illusions.-Examine Fig. 4. and CD appears the longer? CD look longer than A B when they are both the same length ? Draw lines of equal length and at the ends draw forked lines as in Fig. 4, and note the result. Does the attention at the point A, in order to continue along the forked lines, seem to retrace its steps and eliminate a portion of the vertical line, thus shortening it?

B

Fig. 4

Cut a piece of grey paper into pieces, each one inch square. Place these grey pieces at the centre of differently-colored card boards, giving backgrounds of red, yellow, green and blue, respectively, the lower paper or cardboard in each case being not less than three inches square. Place a piece of white tissue paper over all. Note the phenomena. How do you account for the apparent differences in shade of the three pieces of grey paper? Are these differences due to contrast with adjacent objects of different color?

Attentions to Perceptions.-An artist spends days or perhaps weeks in studying one of the great masterpieces of art. Is he simply gaining Sense-perceptions? What more is he doing? Why may a great painting be said to be valuable, not so much for what it is in itself, as for what it stands for (its ideal significance)?

Take some common object, for example, a house, and point out differences between the interpretations which a civilized and an uncivilized man will give of sense stimuli received from the same object? To what is this difference of interpretation due? Give other examples to show that sense-perception is only the first step in the study of objects and that one of the purposes of education is to invest objects with a manifold of meaning over and above mere perception. Consider the following:

In sense-perception there is a response to stimuli coming in contact with the end organ of a nerve, and as a result of this responsive activity certain stimuli are selected from others and interpreted in such a way as to give us knowledge of an object definitely located in space. Sensation is the most elementary form of psychical activity. Sensation and Perception are simply two stages in the same process and cannot be consciously separated. In Perception the sensation is given meaning through interpretation based upon past experience and present attitude. Perceptions are further interpreted and reconstructed into the higher forms of Imagination and Thought, the development being from presentative to representative, from sensuous to ideal.

B. Muscular Activity in Response to Sense Stimuli.

1. Physiological Reflex.-A physician tickled the sole of the foot of a patient whose spinal cord had been seriously injured; the foot was immediately withdrawn. The physician said: "Did you feel when I touched your foot?" The patient replied: "I felt nothing, I only noticed that my foot moved."

Give other examples of muscular activity in immediate response to sense stimulation without any apparent accompanying change in consciousness.

2. Sensation Reflex.-If you inhale snuff under certain conditions, you cannot keep from sneezing. Are you conscious that you have received a sensation and are unable to prevent a muscular response?

Give other examples of conscious but uncontrolled muscular response to sense stimuli.

3. Semi-sensation Reflex.-Compare the spontaneous act of coughing as the result of tickling in the throat with the act of keeping from coughing under similar circumstances.

Give other examples of acts where we are sometimes able to gain control of the response and sometimes fail in our effort.

4. Habitual Reflex.-Walk across the floor and notice the accompanying foot and joint sensations. If you did not receive these sensations, could you walk as expertly as you now do? Why not? Why can you walk and at the same time think of something else? Why could you not do this in childhood when learning to walk? Before you step do you have an idea in your mind of how it will feel when you take the next step? Give other examples of unconscious reflex action through habit?

5. Consciously Controlled Sensory-motor Activity.-Compare the method of giving response in the reaction time experiment outlined on page 23, with the reflexes just considered. Give other examples of conscious muscular response to sensory stimulus.

Is all manual labor sensory-motor?

IV.-SENSITIVENESS AND SENSIBILITY:

Lay your open hand palm upward on a cushion and close your eyes. Have someone lay a small piece of paper or other very light article very gently upon the palm. Have you a corresponding sensation? Under the same conditions, have heavier pieces dropped, until you find the lightest weight

which you can notice. This weight is said to be your threshold for touch, and it measures your sensitiveness.

Now place on your hand a heavier weight, say 13 lbs., and have someone put on an additional half-pound weight. Do you feel any change? Instead of the half-pound weight put on heavier weights until you notice the change. The difference of stimulus necessary to produce a change of sensation is called the difference threshold, and measures sensibility.

Try another weight, say 26 ozs., and notice the difference threshold. Perform a number of experiments to find what relation this difference bears to the weight on the hand.

Experiment similarly for active touch, that is, instead of having the hand rest upon the surface, move it up and down to lift and "heft" the weight.

Speaking generally, the difference threshold is one-thirteenth for passive and one-nineteenth for active touch, whatever be the initial weight.

What would be the effect if additional weights less than the difference threshold were put on very slowly? The foot of a frog has been pressed to a jelly in this way without any apparent pain. Why would it be impossible under normal conditions for anyone, simply by lifting, to distinguish a 12 pound from a 12 pound weight? Can difference threshold be reduced by training? The measurement of sensibility and sensitiveness to intensity of sound may be made by having a very light shot dropped on the table by the experimenter, the observer being stationed some distance away. The faintest sound that can be heard indicates the observer's sensitiveness, and the slightest perceptible difference in sound measures his sensibility, the height from which the shot drops being an index of the intensity of the sound.

The test for just observable differences in the pitch of notes may be made with a tone tester, or with tumblers partly filled with water. Perform these experiments.