by making a study of distant objects, to do more than roughly approximate their actual size and distance away.
In making estimates of this kind we are, in the latter instance, very materially assisted by the peculiar "distance tints" which the mountains assume. The brain becomes accustomed, after a large number of experiences, to associate a certain coloration of objects with certain distances from the eye, and in this way to calculate the distance of an object seen for the first time. Einthoven thinks that the chromatic aberration which even a normal eye exhibits may account for the peculiar colored appearances which distant objects take on.
This explanation is manifestly opposed to the view commonly held, that the minute globules of water in the air act as prisms, and, resolving white light into its component colors, robe the distant mountains in "azure hues." In either case the peaks of the Sierras would deceive the unfamiliar eye, for not only are they more distinctly seen than their fellows of the Atlantic States, but their "distance tints" would entirely mislead the unaccustomed observer.
As the train proceeds rapidly over the level desert my eyes "fix"[1]—i. e., gaze steadily at—a clump of sage-bush which is probably two miles distant. The bush seems to move slowly with the train, while objects between it and my eyes have an apparent motion in the opposite direction. Of these latter the near ones fly past with great rapidity, but the apparent velocity of those farther removed diminishes until, just before the point of fixation is reached, objects come to an apparent standstill. Beyond the point fixed by my eyes objects move in the same direction as the train, their velocity apparently greater the farther away they lie.
Suddenly I shift my gaze from the sage-bush to a large bowlder which is sailing slowly past, probably one thousand yards from the train. Everything is changed at once. The bowlder's retrograde progress is arrested; near objects fly past with accelerated speed; the sage-bush clump forges ahead as if to make up for lost time, while the plain beyond it, indistinct in the distance, races ahead of every object in view. And so I while away a full half-hour, making one conspicuous object after another stand still, go ahead, or sail past at will—all upon the surface of this apparently boundless plain—trying to realize, meantime, that things are not as the moving panorama before me indicates. For, relatively to the train, all objects are passed at an equal rate, the near as well as the distant, those seen by direct as well as those seen
- ↑ When the eye fixes anything, the visual apparatus is so adjusted that the rays of light coming from the object are focused upon the macula, a small central spot in the retina, where vision is most acute; and the object thus fixed is seen more distinctly than surrounding bodies.
