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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

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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.

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FIG. 10.

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

the resulting diplopia produces false orientation. The patient has a false idea of his own position in space with relation to other objects.

It is difficult to realize that objects farther or nearer than the objects looked at are always seen double. For example, if the eyes fix a point (A, Fig. 11) the images A' A' fall on corresponding points

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FIG. II.

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- PHYSIOLOGICAL DIPLOPIA OF OBJECTS FARTHER OR NEARER THAN THE POINT FIXED.

which are end organs corresponding to a pair of neurons, therefore a single impression is the result.

Light from B falls upon end organs belonging to neurons not pairs from the same ganglion cell. In fact, in this illustration they may belong to opposite tracts. The result is two images of B, B'B'.

Demonstration. - Most people find it difficult to see the double images of a single object like a pencil.

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Look at some object on the opposite side of the room, and bring up the two index fingers into the line of vision. Keep the eyes focused on the wall, but notice the fingers. Separate the fingers slightly and a double-ended finger will appear between the other two. This is the composite of the extra image seen by each eye.

Touch perception may be doubled in a similar way. Cross the second finger over the index finger, and then feel of one marble held in another person's hand. A sensation is felt on the side of each finger which normally would necessitate two marbles, and the doubling sensation is very vivid.

The estimation of distance with the two eyes is very much more exact than with one. To avoid diplopia we converge the eyes till the retinal images fall on corresponding points. The nearer the object the greater must be the convergence.

By muscle sense we associate far and near with relatively slight or great convergence.

This arrangement is quite similar to a problem in surveying, where we have given two angles and included side to solve the triangle.

Let A B (Fig. 12), the pupilary distance = the base line. Angles A and B amounts the muscles (M M) must converge the eyes, in order to see C as a single object.

If the object has three dimensions, each eye sees a different picture.

Stereoscopic pictures are right and left like the

retinal images, and when artificially combined by the proper arrangement of lenses and prisms reproduce for us the perception of distance in a landscape

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FIG. 12.—THE BINOCULAR ESTIMATION OF DISTANCE.

with great vividness. After some practise one can manipulate his eyes so as to combine these pictures without a stereoscope. It requires dissociation of accommodation and convergence, and as the normal relation of these two functions is association, it is much better to use the stereoscope than to cultivate this habit.

In this figure (Fig. 13) the smaller circles are decentered toward each other, so in order to fuse these two into one, the eyes must be more strongly converged than is necessary to fuse the larger circles. The sensation is therefore a conic section with the smaller end toward the observer.

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