Fig. 9.—The illusion of the stars.
Take a piece of tinfoil and prick a small hole with the point of a pin. Now when it is dark put a candle behind the tinfoil in such a way that the light comes through the tiny hole. Hold the tinfoil about ten inches from your face, and the hole will appear irregular. If you bring it nearer, it will lose even the least resemblance to a hole and appear as a star! Of course you know perfectly well that it is round, but your eyes have deceived you once more in the same way that they deceive you every starlight night, and the little hole looks something like [Fig. 9]—varying slightly with each individual observer. This deception, or to put it charitably, this mistake of the eyes, is given the very high-sounding name of “irregular astigmatism,” but for all that it is an illusion pure and simple.
Like many well-trained servants, the eyes are quite at a loss if anything contrary to the usual routine is presented to them. They know perfectly well the laws of perspective,—how in the ordinary course of nature these laws are never broken by a hairbreadth. They are therefore accustomed to judge in the fraction of an instant the size of an object by its apparent distance away. That this is the result of practice can be easily seen from the fact that very young creatures—human and otherwise—have no idea of the relative distances of objects, and strain to touch a distant gas-light, or, like a young calf, rush headlong into a neighboring wall which their green young fancy deludes them into thinking is really some distance away. But as we grow older we learn many things, and perspective amongst others.
The Dwarf, the Man, and the Giant
Now if we make a drawing such as [Fig. 10], which represents three men walking down a passage, our eyes know quite well that if all these men were of the same size, Mr. Jones in front would appear smaller than Mr. Smith behind him. And Mr. Smith in his turn would appear smaller than Brown who closes the procession.
Yet in our illustration Jones appears a veritable giant, towering above Smith and making Brown appear a mere pigmy. If you measure them, you will find they are all three the same size.
The reason of the deception is this. The lines showing the passage disappearing into the far distance immediately suggest to the eye the correct perspective, and, knowing the laws of that perspective, the eye is perfectly convinced that if all three were the same size, Brown in the rear would appear proportionately bigger than Jones. As he does not do so, the eye immediately leaps to the conclusion that he must be very much smaller. It therefore telegraphs to the brain that Brown is a dwarf, following in the tracks of an ordinary man and a giant!
Fig. 10.—The dwarf, the man, and the giant.