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CHAPTER IV

SUMMARY

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The blind spots; the two optic discs. Retinal shadows: erect, because cast by objects too near the eye to form image on retina; outward projection of, produces inverted image.. Outward reference of tactile sense; flexible cane. lar estimation of distance. — Difficult without parallax. — Law of corresponding points. Each ganglion cell two neurons, divide for both retinæ. - Orientation, with prism.-Diplopia : — physiological at distances farther or nearer than point fixed. - Analogy of digital tactile sense. - Binocular estimation of distance. Fusing successive double images. — Coördination of convergence and accommodation. - Stereoscopic perspective. — Pictures correspond to right and left retinal images. Convergence required to fuse, determines distance; convergence excessive, nearness; convergence slight, distance. - May overcome mathematical perspective. - Binocular vision an acquired faculty. - Fusion training.—The amblyoscope. Phoro-optometer stereoscope. Controlled reading.

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THE BLIND SPOTS

EACH optic disc, which is the head of the nerve, is a blind spot, but the fact escapes our attention because ordinarily the image cannot fall on both at the same time, so that one or the other eye can always see every point in the binocular field. Indeed, if only one eye be used, the blind spot is so small and eccentric that it is never noticed.

To demonstrate in right eye. (Fig. 5.)

Close left, hold paper so that 2 is directly in front

of and on level with the right, about eight inches away. Fix the sight on 1 and bring the paper slowly

FIG. 5. DIAGRAM TO DEMONSTRATE THE BLIND SPOT.

toward face. At about five inches distant 3 disappears, reappearing again at about four inches. The figure may be reversed for left eye.

RETINAL SHADOWS

Objects too near the eye to be focused on retina cast shadows which are erect, but the outward reference of a direct shadow gives an inverted image in space.

Demonstration (Fig. 6). Hold pin, head up, so close to eye that it touches the lashes. A visiting

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FIG. 6. THE INVERTED RETINAL SHADOW.

card with pinhole perforation is now brought up with hole in line with head of pin. The object is seen in space beyond the card, that is, the shadow is referred to the place which an object would occupy

normally to produce such an image on the retina. This is indicated in figure by broken lines.

Outward reference of tactile sense may be made to extend beyond the finger.

Demonstration. With a flexible rattan cane one should feel his way about the room blindfolded. The sense of feeling the point of contact between cane and wall or floor is very vivid. In the same way the surgeon feels with the point of probe or knife.

Estimation of distance depends upon the refinement of muscle sense. It is both monocular and binocular. That this is not a congenital faculty is illustrated by the child reaching for the

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FIG. 7.
- MONOCULAR ESTIMATION
OF DISTANCE BY PARALLAX.

him." Objects were seen in their proper direction, but he could not distinguish between far and near.

To illustrate our dependence on parallax, with one eye closed, attempt to bring the finger down on the point of a pencil held by another. If this is held against a side wall the parallax makes the estimation easy.

Let A (Fig. 7) be the pencil. We learn by expe

rience just how much contraction of the muscle M is required to turn the eye so as to "fix" 1, 2, and 3, respectively, and so determine distance by this muscle sense.

A good method of eliminating parallax is to look at a coin on a table with the eye a little below the level of the table, so that the edge of table and coin are visible, but not the top of table. (Fig. 8.) As before, the other eye must be closed and the coin

FIG. 8.- DIAGRAM TO ILLUSTRATE DIFFICULTY OF ESTIMATING DISTANCE WITHOUT PARALLAX.

must be placed by another person after the eye has taken the correct position. Several trials will be necessary to bring the finger down on the coin.

Binocular single vision depends upon the law that images falling on identical portions of the two retinæ cause the sensation of one object.

This is because each ganglion cell has two neurons which run together in the optic tracts, but part company at the chiasm, one going to the outer half of the retina of the same side and the other crossing over to the inner half of the retina of the other eye. (Fig. 9.)

Pathology furnishes additional proof of this, in cases of hemianopsia or one-sided blindness. Here

a lesion of the right optic tract will cause loss of the left field of both eyes.

Orientation. — If a prism of six diopters be held

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FIG. 9. THE PARTIAL CROSSING OF THE FIBERS OF THE OPTIC NERVES.

base down before one eye, diplopia results, because the prism deflects the light from the object to some other than the corresponding point.

FIG. 10.

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FALSE ORIENTATION PRODUCED BY PRISM.

The image will fall on the lower part of the retina, and the object will be referred to a point above, which would normally excite that portion of the retina. This is called false orientation, and is exactly what happens when the inferior rectus is paralyzed, and

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