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FIG. 25.

viz., between the central spot and the blind spot. Now, as the paper comes nearer, the eye turns more and more in order to regard A, the image of B travels slowly over the retina noseward until it reaches the blind spot, and the object disappears. As the paper still approaches, the image of B continues to travel in the same direction until it crosses over the blind spot to the other side, when the object immediately reappears.

The accompanying diagram, Fig. 25, illustrates this phenomenon. Let A and B represent the two objects, and R and L the positions of the right and left eyes respectively. The right is drawn, but the left, being shut, is not drawn, but only its position indicated by the dot. The central spot is represented by c, in the axis A c, and the blind spot by o, where the optic nerve enters. It is obvious that the image a of the object A will be always on c, and the place of the image of B is on the intersection b of the line B b with the retina.

R3

R2

R'

Now, as the eye approaches the objects A and B, it is seen that the image b of B travels toward the blind spot, o. At the second position of the eye, R', it has not reached it. At the third position, R", it is upon it.

F

At the fourth position, R'"', it has already crossed over and is now on the other side. At the third position, R", the object B disappears from view.

The distance at which the disappearance takes place will, of course, depend on the distance between the objects A and B. If these are 3 inches apart, then the disappearance on approach from a greater distance takes place at about 1 foot, and the reappearance at about 10 inches. If the objects be 1 foot apart, then the disappearance takes place at 48 inches, and the reappearance at 38 inches.

Experiment 2.-Place a small piece of money on the table. Shutting the left eye, look steadily with the right at a spot on the table a little to the left of the piece, and move the piece slowly to the right while the point of sight remains fixed; or else, the piece of money remaining stationary, move the point of sight slowly to the left. At a certain distance from the point of sight the piece will disappear from view. Beyond this distance it will reappear.

Experiment 3.-The experiment may be varied in many ways. If, when the object B has disappeared from view in the previous experiments, we open the left eye and shut the right, and look across the nose at the object B, then A will disappear. Thus we may make them disappear alternately. If, finally, we squint or cross the eyes in such wise that the right eye shall look at the left object A, and the left eye at the right object B (the two, A and B, had best be similar in this case), then B will fall on the blind spot of the right eye and A on the blind spot of the left eye, and they will both disappear; but a combined image of A and B on the central spots of the two eyes will be seen in the middle. This, however, is a phenomenon of bin

ocular vision, and will be explained farther on (see page 107).

Experiment 4.-Any object, if not too large, may be made to disappear by causing its image to fall on the blind spot. For example: From where I now sit writing the door is distant about 10 feet. I shut my left eye and look at the door-knob. I now slowly remove the point of sight and make it travel to the left, but at the same level; when it reaches about 3 feet to the left, the door-knob disappears; when it reaches 4 feet, it reappears. Precisely in the same way a bright star or planet, like Venus or Jupiter, or even the moon, may be made to disappear completely from sight.

Size of the Blind Spot.-As every point in the retina has its representative in the visual field, it is evident that the size of the invisible spot is determined by the size of the blind retinal spot. We may, therefore, measure the latter by the former. I have made many experiments to determine the size of the invisible spot. At the distance of 3 feet (42 inches) I find the invisible spot 12 inches from the point of sight, and 3 inches in diameter; i. e., a circle of 3 inches will entirely disappear at that distance. Taking the nodal point of the lenses or the point of ray-crossing at of an inch in front of the retina (it is a very little less), an invisible spot of 3 inches at a distance of 3 feet would require a blind retinal spot of a little more than inch in diameter. At 36 feet distance the invisible area would be 3 feet; it would cover a man sitting on the ground. At 100 yards distance the invisible area would cover a circle of 8 feet diameter. In a word, the angular diameter of the invisible spot is a little more than 41°. Helmholtz makes it a little larger than this.

Representative in the Visual Field of the Blind Spot.Since every condition of the retina has its visible representative in the field of view, it may be asked, "If there be a blind spot, why do we not see it, when we look at a white wall or bright sky, as a black spot, or a dusky or dim spot, or a peculiar spot of some kind?" I answer: 1. With both eyes open there are, of course, two fields of view partly overlapping each other. Now the invisible spots in these two fields do not correspond, and therefore objects in the invisible spot of one eye are seen perfectly by the other eye, and hence there is no invisible area for the binocular observer. But it will be objected that even with one eye we see no peculiar spot on a white wall. I therefore add: 2. That we see distinctly only a very small area about the point of sight, and distinctness decreases rapidly in going from this point in any direction. Therefore the correspondent or representative in the field of view may well be overlooked, unless it be conspicuous, i. e., strongly differentiated from the rest of the general field. 3. But if this were all, close observation would certainly detect it. The true reason is very different, and the explanation is to be sought in an entirely different direction. Writers on this subject have expected to find a visible representative, and have sought diligently but in vain for it. But the fact is, they ought not to have expected to find it. The expectation is an evidence of confusion of thought -of confounding blackness or darkness with absence of visual activity. Blackness or darkness is itself but the outward projection of the unimpressed state of the bacillary layer; but there is no bacillary layer here. We might as well expect to see a dark spot with our fingers as in the representative of the blind spot. A black spot, or a dark spot, or a visible spot of any kind, is

not the representative in space of a blind or insensitive retinal spot. The true representative of a blind spot is simply an invisible spot, or, in other words, a spot in which objects are not seen. If we could differentiate it in any way, it would be visible, which it is not. As it can not be differentiated in any way, the mind seems to extend the general ground color of the neighboring field of view over it. This is, however, a psychological rather than a visual phenomenon. It is for a similar reason that it is impossible to see any limit to the field. of view, except where it is limited by the parts of the face, as nose, brows, etc. There is a certain limit horizontally outward where vision ceases, but it is impossible to detect any line of demarkation between the visible and the invisible.

3. Erect Vision.-Retinal images are all inverted. External images or signs of objects are outward projections of retinal images. How, then, with inverted retinal images, do we see objects in their right position, i. e., erect? This question has puzzled metaphysicians, and many answers characteristic of this class of philosophers have been given. The true scientific answer is found in what is called the "law of visible direction." This law may be thus stated: When the rays from any radiant strike the retina, the impression is referred back along the ray-line (central ray of the pencil) into space, and therefore to its proper place. For example: The rays from a star (which is a mere radiant point) on the extreme verge of the field of view to the right enter the eye and strike the retina on its extreme anterior left margin; the impression is referred straight back along the ray-line, and therefore seen in its proper place on the right. A star on the left sends its rays into the eye and strikes the right side of the retina, and the

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