Page:Popular Science Monthly Volume 40.djvu/640

620 by the alert ocular muscles. When, however, the object is too near the eye, or when its motion is too swift, the muscles are not quick enough in their action to preserve this delicate state of optical rest; the image is thrown across the retina, and the object is seen to move. A glance at this diagram (Fig. 3) will show how these retinal impressions are received and interpreted.

The first figure is intended to represent three objects seen from a train in motion. Although the middle one is fixed by the eye, and is consequently most distinctly seen, the blurred images of the other two also fall upon the macula, so that for a single instant they are all optically at rest. A moment later, the eye, still fixing the middle object, has moved from 1 to 2, and, as is seen in 2, the images corresponding to the near and the remote objects have passed over the retinal area. Motion in the opposite direction is, according to the law just laid down, attributed to each, while the middle object still gives the impression of comparative rest.

When, however, the image of the moving object is kept fixed upon the macular region, the eye may judge of the rate of its motion by the amount of effort put forth by the ocular muscles necessary to keep the image focused upon the macula. This method of calculation is defective, and gives rise to numerous optical errors. For example, the movement of a lady's fan in front of her face, the velocity of a base-ball through the air five hundred yards off, and the rate at which the night express travels as it approaches "end on"—its head-light gleaming in the distance—would all be incorrectly calculated if the brain were to accept ocular evidence alone and based on one or both of the foregoing rules. The to-and-fro movement of the fan would be interpreted as exceedingly quick; the velocity of the base-ball would be next in order; while little or no motion would be attributed to the approaching train.

Becoming tired of looking at the wayside scenery, I find myself, in a sort of brown study, watching the back of the plush-covered seat in front of me, and then I discover that the retinal