Page:Popular Science Monthly Volume 1.djvu/487

Rh the eye—the yellow spot marking the entrance of the optic nerve. All perception whatever is arrested within the bounds of this spot—it is a blind point in our field of view.

The blind spot is by no means so excessively small. At the distance of four paces, it would cover a man's head in the centre of your field of view; and almost a hundred moons in the sky would find room within its bounds. When Mariotte made this important discovery, it caused so much commotion that the experiment had to be repeated before the King of England in 1688. In the endless variations of the experiment, the remarkable fact only received a new confirmation. For the rest, this discovery had almost proved fatal to the doctrine of perception; for, as at that time the optic nerve and the retina being considered as essentially the same, one might deduce, a priori, the inference that, just at that point of entrance at which all the conducting fibrils converged, a heightened sensibility might be argued. But, now proving to be insensitive, the retina itself could no longer be regarded as the regular conductor of the sensation of light. And this was the conclusion to which Mariotte did arrive, transferring the sensibility to the choroid behind the retina, till at length Bernoulli and Haller again restored it to its rights.

This apparent enigma is explained by what I told you of the general relation of light to the visual organ. The part the optic nerve plays is only that of a conductor, while the sensations of the vibrations of the ether, as also of the specific sensory irritation, is committed to the retina, or, more correctly speaking, to its external layer.

Another question is, Why does the existence of the blind spot usually escape our attention? The chief reason is that, as the gap is regularly situated in the same spot in the field of view, the idea has learned to fill it up in the most natural manner, and as is suitable for the connection of objects. For instance, I draw the figure of a cross on the board, and fix my eye on it, so as to cause the centre of this figure enclosing the point of intersection to fall on the blind spot; in this manner I believe indeed that I see a cross, while in reality I only see what lies beyond; fancy supplementing the rest. The cross is a commonly-known figure, and when any two lines take a perpendicular direction toward each other, they as a rule really do intersect each other. The best proof of this being the case is that, when you obliterate all that lies within the district, you still continue to see the cross; and, to make the experiment more elegant, if you place some photograph in the empty space, you do not perceive it, you still continue to see only the cross. You have here, then, a conjunction of objective sensory action and subjective influence, apparently with the help of the central extremity of the optic nerve, which is highly significant for the whole doctrine, and which to a certain degree combines what I have been endeavoring to explain to you on both of those branches of the subject.