THE INVERTED IMAGE

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... The theory," according to Cosmos, "makes it easy to understand the mechanism of the reinversion of the retinal image. The double curve effects a complete reversal of the order of the nerve fibers both from top to bottom and from right to left, the two half-turns being exactly equivalent to a halftwist or rotation thru 180 degrees about the axis of the bundle."

Individual anatomical variations of all parts of the human body are frequently discovered by surgeons. The location and number of the branches of the arteries, veins, and nerves are not the same in different individuals. Is it reasonable to presume that the optic nerves are an exception to this rule? If not, then there are many people whose optic nerves do not twist the exact 180 degrees required.

If erect vision depends upon this condition, it is evident that a faulty or anomalous development would furnish instances of partial or complete inverted vision. Very many cases of this kind are needed to substantiate this very ingenious theory, and none are given. Moreover, the theory is based on the misconception that we see the image, not on the retina, as the tactile reinversion theory presumes, but at some other portion of the cranium. In a very recent work Doctor Souter says: "It is apparent that the retinal image is always inverted with respect to the object of vision. The mind, however, takes no cognizance of this inversion, since it possesses the power of exter

nal projection so that we see not the image but the object in its true position." I

Then the writer proceeds to repeat the old argument of tactile reinversion as follows: "This power has doubtless been derived thru association with the sense of touch. We have learned that a stimulus conveyed to the brain from the upper part of the retina proceeds from an object situated below the eye, and vice versa, and that a stimulus on the temporal side of the retina must proceed from an object on the nasal side of the eye, and vice versa."

'The Eye and Nervous System. Posey and Spiller. Lippincott, April, 1906, p. 35.

CHAPTER IV

SUMMARY

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. - Monocular estimation of distance. - Difficult without parallax. — Law of corresponding points. Each ganglion cell two neurons, divide for both retina. 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.

THE BLIND SPOTS

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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 I and bring the paper slowly

2

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

Өн

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

moon.

When Doctor Sidis' patient, Mr. Hanna previously referred to, first

opened his eyes,

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a great flat pic

ture was before

Ο

A

M

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