6i8 THE POPULAR SCIENCE MONTHLY. 



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. Eintho- 

 ven 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 dis- 

 tant 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 " * — i, e., gaze steadily at — a clump of sage-bush which is prob- 

 ably two miles distant. The bush seems to move slowly iviili 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 bowl- 

 der 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 accel- 

 erated 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 aj^par- 

 ently boundless plain — trying to realize, meantime, that things 

 are not as the moving panorama before me indicates. For, rela- 

 tively 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 surround- 

 ing bodies. 



