Strange as it may seem, upon certain occasions, the best eyes are actually blind! If you shut one eye and hold the page with [Fig. 1] at arm’s length, you will be able to see both the spots A and B. Now look steadily at A, and you will still see B quite plainly, but if you gradually draw the book nearer to your eye, a certain point will be reached when B becomes invisible, although if you continue to make the book approach your face B will spring into view once more. In other words, at the moment when you could no longer see B your blind spot had been directed towards it, and of course saw nothing.
Fig. 1.—When two are one.
Fig. 2.—Section of the eye.
No doubt you would like to know where this blind spot is, and why our eyes should possess such a thing. [Fig. 2] shows the section of an eye which can be explained in very simple terms. The thick black line A is a sheet of nerves which entirely envelops three-quarters of the eye, and meeting in a point at E passes upwards into the brain, where it records what the eye has seen. The light enters between the points C C, the iris, and striking through the lens B throws all objects within the scope of vision upon what is called the retina or screen, D. Now this screen is furnished with millions of little nerves, each one of which records on the large nerve A whatever is thrown upon it, and all these records are gathered together by A and passed up to the brain.
But at the spot E, where these big nerves are collected together, the retina, as you notice, is pointed, and gives no record of what is thrown upon it. So, you see, when any object happens to come into such a position with the eye that its image is cast upon the point E of the retina, we have no record sent to the brain—in other words, we cannot see it.
But the eye is not only blind in one point; it is very apt to be deceived by appearances, and to make all kinds of mistakes in consequence. Take [Fig. 3] for instance. Would you not say that B D is shorter than A C? Yet if you measure them you will find they are the same length. Or in [Fig. 4], A B is surely longer than C D. They are identical. Or take [Fig. 5], A is clearly farther from B than C is from B, and yet A B and B C are of the same length.
Fig. 3.—Is A C longer than B D?