therefore fatigues with embarrassing rapidity. The colors we ordinarily see are all mixed, and the yellow or blue element in a mixture is more stable than the red or green, for reasons explained under Exp. 2; hence the latter fatigues more quickly and falls out, leaving the other dominant. This is the goal of the first stage. Then follows the process of adaptation, as in afterimages, when the stable color element becomes fatigued, and leaves a blank or gray. Then follows the last stage, which corresponds to the after-image of the strongest element in the first stage.'

*

The fact of order in these changes is perhaps as astonishing as their rapidity. Our common experience of these changes in daily life has resulted simply in a distrust-both conscious and unconscious of indirect color-vision. When we think of it, we think of it as chaotic; when we inadvertently follow it, we feel uneasy; in normal perception we automatically neglect it.

These four experiments may suffice to give a glimpse of the complexity and wonderful arrangement of the color-fields and to point the way in which the scientific attitude is rewarded by revelation of system and reason

tion does not last long. A process of regeneration takes place within the organ; and so long as the retinal regeneration continues, one sees color whether the colored object be present or not. This fourth stage of the visual process is analogous to that which you studied in your experiments with after-images. The visual sensation which now appears in indirect vision is complementary to the color which was present in the second stage." * It is remarkable that after-images are never observed in the periphery of the dark-adapted retina, and very rarely more than 30° from the fovea. They may be observed, however, in the periphery of the light-adapted retina.

in it all. Many of the problems of indirect color-vision have not even been mentioned, and conditions and variables have been stated in the barest way possible.

In conclusion, let us notice a beautiful biological arrangement. If we compare the direct with the indirect field with reference to sensitiveness, we find that we are most sensitive to color and form in the direct field, and to light and movement in the indirect field. That is, we can see color and form most accurately when the image falls upon the central region of the retina, but we can detect movements and changes in brightness more readily when the outer-lying portions of the retina are stimulated."

This is a story of adjustment and it suggests to us the real office of the indirect field. The central region of the retina, corresponding as it does to the direct field, is the organ of attention, of concentrated mental activity, which represents the environment in terms of space and color; while the indirect field is merely accessory. Its functions are those of a scout or guardian. The life-preserving value of this arrangement is clear. Consciousness is warned of the presence or approach of an object beneficent or noxious to life, by impressions of luminosity or movement in the indirect field. If then the signal is heeded, the eye quickly turns so as to bring the object of scrutiny into the direct field where its true nature can be seen accurately, by the most efficient and economical expenditure of energy.

CHAPTER IV

VISUAL SPACE

For One.*

ALTHOUGH We have several space-senses, most of us live predominatingly in a world of visual space. Visual space-perception is therefore one of the largest and most important topics in psychology. We shall limit our experiments in this chapter to a few features of visual space images.

1. Outward Projection of the Visual Image. a. Floating Flakes.-Look toward the sky with your eyelids almost closed and observe a sort of snowfall effect. Describe it.

These flakes are the shadows, on the retina, of particles floating in the vitreous humor. They are projected as objects in outer space in accordance with the law of outward projection.†

*This chapter presupposes knowledge of the structure and function of the eye, as outlined in the text-books on psychology. "We see indistinct motes floating about in the field of view and slowly gravitating downward. Sometimes they are undulating, transparent tubes, with nucleated cells within; sometimes they are like inextricably tangled threads, or like matted masses of spider's web; sometimes they are slightly darker spots, like faint clouds." (Le Conte.)

b. Shadows of the Retinal Blood-vessels.*-Standing in a dark room with one eye closed, wave a candleflame or burning match gently in a small circle close to the side and slightly downward and forward from the other eye; look at the opposite wall and you will see a network like Fig. 3.†

Describe size, distance, color if any, stability, etc.

The retinal blood-vessels enter at the same point as the optic nerve and spread in a network inside of the retina. As the rods and cones lie back of this layer, the blood-vessels cast shadows which become visible under the prescribed conditions of illumination. There is nothing on the wall to correspond to this system, yet you see it distinctly out there in space, in accordance with the law of outward projection.

We are never directly aware of the retinal impression.

* Perform this experiment in the evening, if necessary.

Do not fatigue the eye. Nothing is gained by straining it. If you do not get the effect at once, it is because it seems unreasonable to look for such a thing. Look for a network like Fig. 3, very much enlarged, and you will see it. The experiment should not require more than a minute or two.

We always see objects out in space. The retinal impression is automatically referred to its normal source: that is the law of outward projection. The history of the evolution and development of this tendency constitutes one of the most important chapters in psychology as well as in the theory of knowledge.

The conditions of the above experiments were unusual, if not unnatural, but the mind responded in its habitual way, and this misdirected tendency revealed to us something of the nature of the normal process. The image was laid bare, as it were, by the absence of the object.

The retinal light furnishes us another illustration of this class of entoptic phenomena.* But the best and most serviceable illustration of all is the after-image, with which we are already familiar. It represents a physiological condition of the retina but is always seen out in space, never within the eye.†

*To observe the retinal light, go into a dark room and cover your eyes so as to shut out all possibility of objective light. Behold, in a moment you see a gorgeous array of colors in front of the eye. They tend to grow brighter, usually fashion themselves into fantastic designs, and are in a continual kaleidoscopic commotion. These are nothing but the projection of the local irritation of the retina, chiefly through the circulation of the blood. These retinal lights are the stuff from which many visual dreams are "made".

"Seeing stars" from a blow or fall illustrates the same principle. The story is told of a man who was attacked and knocked senseless by a blow on the temple during a pitch-dark night. He accused a neighbor whom he had to confront in court with the evidence, which was essentially this: "It was pitch-dark, but the moment I felt the blow there was a great flash by the light of which I saw my assailant." This may have been naïve testimony: he had seen a light at the proper time; he suspected his neighbor; his conviction was that he had seen his neighbor.