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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

B

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FIG. II.

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.

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.

Decenter the small circles the other way (Fig. 14) and we reverse the position of the cone.

It has been shown by Worth, of London, that

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FIG. 13. Diagram TO DEMONSTRATE BINOCULAR PERSPECTIVE; THE SMALLER END SHOULD APPEAR TO BE TOWARD THE OBSERVER.

the movements of the two eyes are largely controlled by a fusion sense, and that a faulty development of this faculty is frequently the cause of cross eyes. If taken early many of these cases can be cured without operation by developing the fusion faculty.

For this purpose he has devised an instrument called the amblyoscope, with which even a cross

DIAGRAM TO DEMONSTRATE

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FIG. 14. BINOCULAR PERSPECTIVE; THE LARGER END SHOULD APPEAR TO BE TOWARD THE OBSERVER.

eyed person can fuse simple images. This he cannot do with the ordinary stereoscope.

This instrument does not allow of one's watching the eyes while they are being exercised. In order I Squint: : Its Causes, Pathology, and Treatment.

to observe the motions of the eyes and make it possible to measure exactly the deviation during binocular vision, the author has devised a stereoscopic attachment to a well-known instrument called the phoro-optometer. With this instrument the strength of the prisms may be varied to suit the individual case.

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FIG. 15. AUTHOR'S STEREOSCOPIC ATTACHMENT TO THE PHORO-OPTOMETER.

In order to ensure the use of both eyes for one's reading, writing, etc., and other near work, the author has contrived a control device which consists of a band of blackened aluminum, held by a headband midway between the eyes and the printed page. The fields seen by the two eyes overlap, and if binocular vision exists one experiences no difficulty in reading thru the obstruction, but if either eye is

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