EACH optic disc, which is the head of the nerve, is a blind spot, but the fact escapes our attention be-cause 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.
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 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-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 A 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.É Hanna previously referred to, first opened his eyes, M ” a great flat picture was before 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 experience just how much contraction of the muscle M is required to turn the eye so as to ” fix” I, 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 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 retinae 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 base down before one eye, diplopia results, because the prism deflects the light from the object to some other than the corresponding point.
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 10 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. I2), 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 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 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- 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 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.
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 head-band 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 suppressed the band obscures some of the letters in each line.
Without entering into a discussion of the peculiar mental condition which causes a cross-eyed person to suppress the image seen by the squinting eye, the fact is here mentioned as an illustration of the intimate relation between psychology and medicine.
Le Conte says : ” As a means of scientific culture the study of vision seems to me almost exceptional. It makes use of and thus connects the sciences of physics, physiology, and even psychology. It makes the cultivation of the habit of observation and experiment possible to all; for the greatest variety of experiments may be made without expensive apparatus, or indeed apparatus of any kind. And, above all, it compels one to analyze the complex phenomena of sense in his own person, and is thus a truly admirable preparation for the more difficult task of analysis of those still higher and more complex phenomena which are embraced in the science of psychology.”