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the objects be 1 foot apart, then the disappearance takes place at 48 inches, and the repearance 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 first experiment, 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 binocular vision, and will be explained farther on (see page 131).
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 31 feet (42 inches) I find the invisible spot 12 inches from the point of sight, and 34 inches in diameter; i. e., a circle of 37 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 31 inches at a distance of 31 feet would require a blind retinal spot of a little more than a 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 4° 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,
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 représentative 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 demarcation between the visible and the invisible.
But if we can not see the representative of the blind spot-i. e., the invisible spot-we can under certain conditions detect its exact place in the field. The phenomenon now about to be described can not be seen during the day when the retina is constantly stimulated, and therefore less sensitive, but may be easily observed on waking up in the middle of the night, or in early morning when the retina is exceptionally sensitive.
Experiment 5.-If on first waking in the morning the lids be closed and the eyes be turned quickly and strongly to one side or the other, as if to look at a point on the extreme verge of the visual field, two brilliant circles of radiating lines surrounding each a blank space are momentarily seen flashing out in the dark field on each side of the point of sight. On turning the eyes strongly in the opposite direction they again flash out on the dark field on the other side, at the moment of extreme strain of the ocular muscles. The phenomenon is especially brilliant if the visual plane be lowered or turned toward the feet. In Fig. 32, the curve represents the spatial concave. The eyes are shown turned strongly to the right and directed on Ps, the point of sight with the bright circle on each side. The dotted lines show position of the eyes turned to the left and the place of the bright circles.
The phenomenon is really extremely brilliant and conspicuous; but on account of its flashing momentariness, and still more on account of the position of the circles a little removed from the point of sight, where alone form is given accurately, it is difficult to make an 'exact picture. In Fig. 32, I give it as nearly as I can.
Now there can be no doubt that we have here indicated the exact place of the invisible spot. The blank spaces from which the bright rays diverge are the
representatives of the blind spots or places of entrance of the optic nerves, and the circles of bright rays are the representatives of the immediately surrounding bacillary layer. The parts surrounding the invisible spot are differentiated both from it and from the general field of darkness, and thus the place of that spot is exactly indicated. The cause of the phenomenon is obviously the strain on the optic nerves by pulling and bending, in the quick and violent turning of the eyeballs.