Page:Popular Science Monthly Volume 40.djvu/639

 by indirect vision. But, in looking from my car window, I am made the subject of optical illusions common in a journey of this sort.

Notwithstanding the many wonderful things about the mechanism of vision, it exhibits, after all, a great many crudities. Intellectually, for instance, the optic centers are low down in the scale of origin. Even the olfactory nerves have a higher cerebral origin than they. Accordingly, we often find them committing all sorts of errors, from whose consequences only the experience of the other organs (acting as special detectives) enables the organism to escape.

Simple "seeing" ought not to be followed, in all cases, by implicit belief. When, for example, as in this case, the eye forms part of a moving mass, the motion is wrongly attributed by the optic centers to surrounding bodies. The explanation of how this comes about is easy when one considers certain facts in elementary optics. If I close one eye and slowly move a pen from right



to left a few inches in front of the other eye, the direction of the movement is rightly interpreted by my brain, although by a reference to Fig. 2 it will readily be seen that the retinal image of the pen moves in an opposite direction over the background of the eye.

Precisely the same effect is obtained if, instead of moving the pen, I look straight forward and move my head from left to right, simply because the same impression is produced—i. e., the retinal image moves from left to right.

When, therefore, the image of an object is made, it matters not how, to move over the retinal background, motion in an opposite direction is immediately referred to the object itself. It makes no difference, then, so far as the optical effect is concerned, whether the solid plain with the objects on its surface be carried past the observer at rest, or whether the observer himself move past or over the plain. Further, when there is no movement of the image over the retina, no motion is detected by the eye; optically, the object is at a standstill. That a body moving in front of the eye should appear to be stationary, its image must always be kept in the same position on the retina. This is accomplished