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The experiments have all been devoted to color and brightness. The phenomena of visual contrast might have been equally well illustrated in visual perception of space. The general law is that opposites enhance each other although the actual explanation for this may be different in different cases. As black enhances white and red enhances green, so there is a reciprocal enhancement between the long and the short, the large and the small, the narrow and the wide, the irregular and the regular, the dull and the sharp, the smooth and the rough, the straight and the crooked, the ugly and the beautiful, etc. When the tall and the short man walk together, the tall one looks taller and the short one looks shorter than otherwise. A poor penman is mortified to see his signature together with the signature of a good penman. A pocket-knife is dull in comparison with a razor, but sharp in comparison with a hatchet.

Similar illustrations might be found in the time, or duration, of visual acts. Contrast operates in all the attributes of sensation-quality, intensity, duration, and space. And we find it in all the senses. Indeed, one of the laws of contrast is that it is strongest in those senses with which we make the poorest discrimination. Hence the most striking illustrations of contrast are found in the lower senses, as in taste, smell, and temperature.

CHAPTER III

THE VISUAL FIELD

For One.

THE problem is to measure the field of vision for white and colors, and to determine some characteristic color-changes in the indirect field.

Make the following preparation for the experiment: Lay a piece of cardboard back of Fig. 1. Prick through the page with a pin at each number on the arc, at the free end of the heavy line, and at the principal points in the light line. Trim the card by cutting according to the tracing of the light line. Insert the degree-numbers at the appropriate points. Draw the heavy line. Punch a pinhole at the free end of the heavy line, which is the center of the arc. Put a thread through the hole and tie a knot at the back of the card. Tie a knot at the other end of the thread, fifty centimeters away. Take the large black square and the small white, red, green, yellow, and blue squares from the book envelope. Lay the white square upon the large black square as a background; then stick a pin through the knot at the free end of the cord, through the two squares near a corner of the small one, and finally through a cork

which may serve as a handle. The quadrant and ob ject-card thus arranged may be called a perimeter.

*

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The perimeter is a simple means of measuring the direction of an object in the indirect field of vision.

*There have been two general types of instruments employed in perimetry of vision: (1) those in which the measurements are made upon a plane surface, and (2) those in which the measurements are made upon the arc of a circle centered at the eye. The former is called a campimeter, the latter a perimeter. The present outfit works on the perimeter principle. The most effective perimeter is an instrument with colored lights in a dark room.

Seat yourself in good reflected light with the back toward the source of light. Mark a dot on a piece of white paper and fasten it up sixty centimeters directly in front of the eyes. Blindfold one eye; hold the quadrant with one edge close to the other eye so that in looking straight forward at the dot, the eye sights along the heavy straight line on the quadrant. When the eye is fixed upon it, the dot becomes the fixationpoint, or point of regard, and is said to lie in the direct field of vision. The visual space around it is spoken of as the indirect field.

1. The Field for White.-Hold the quadrant in the horizontal plane in front of the right eye so that the regard-line on the quadrant points exactly toward the fixation-point when you sight along it. Keep the head upright and firm. Fixate the dot which is the point of regard and do not allow the eye to wander away from it during the actual trial. Move the object-card inward from the extreme right until the white disk can first be seen as white.* The thread being held taut will indicate the number of degrees from the line of regard. Record this. Make five trials, and find the average and the mean variation for these.†

Proceed in the same manner and measure along the other three cardinal radii, namely, with the white entering from below, from the left, and from above.

*Move inward at such a rate that the destination is reached in about eight seconds. Take special care that the head does not turn or the eye wander from the point of regard, and that the point of regard is directly in front of the eye.

The mean variation (m. v.) is a measure of the degree of agreement in a series of records. It represents the average of

To represent these results graphically, draw four radii from a point representing the point of regard— one to the right, one to the left, one down, and one up, and place a dot on each radius to represent the corresponding measurement on the scale of one millimeter to one degree. Mark each of these dots w.*

These results are stated with reference to the field of vision; they might equally well have been stated in terms of regions on the retina. The outer or temporal field of vision corresponds to the inner or nasal region of the retina, and the upper field of vision corresponds to the lower region of the retina. The temporal field of vision is larger than the nasal, and the lower is larger than the upper. This difference is due to the limitations placed by the nose, cheek-bone, and brow.t

the deviations of each individual record from the average of all the records for the group, regardless of sign. Thus

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*Thus, if the white square was first seen as white at fifty degrees above the eye, put a dot fifty millimeters from the center on the appropriate radius.

+ Supplementary Exercise.-There is a totally blind spot in the nasal region of each eye. It is located at the point of entrance of the optic nerve, about fifteen degrees from the fovea, or point of clearest vision. If this exercise is assigned, the student should devise his own methods and means for one or more of three exercises: (1) to locate the blind spot; (2) to survey and determine its shape and area; and (3) to determine how it is filled out in perception.

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