Rensselaer Republican, Volume 16, Number 48, Rensselaer, Jasper County, 7 August 1884 — Optical Illusions. [ARTICLE]

Optical Illusions.

Place a man and a dog side by side at a distance of twenty feet, and an J person with an eye capable of distinguishing them will be able to tell which is on the right, which on the left. The eye is not easily deceived as to position at fight angles to the line vision. Let the man advance five feet; it is easy to tell that the dog is farther away than the man. Next, place the man at a distance of 100 feet, tlie dog. at 105 feet; it is not so easy to decide as before, although mistakes are rare with a normal eye. But at 500 and 600 feet, respectively, it is less easy, although we can still tell which is to the right and which to the left. The images “Vbrmed on the retina by the same object at different times are very similar, differing only in size and ditinctness. For this reason it is difficult to judge of distances, requiring much practice. A person standing on a straight strip of railroad is rarely able to tell whether a distant train is approaching or receding, or at rest, so slight is the change in apparent size from wfyichthe distance is to be estimated. Upon the sea it is very difficult, without long practice, to judge of distances, 'Refraction always changes the apparent place of an object, so that we seem to see the sun after it has gone below the horizon. A more striking but less frequent phenomenon of refraction is that known as mirage. Refraction also affects the color of an ob- : ject. The media through which the light passes has more or less effect upon the ray. In a f g objects are dimly seen, the effect resembling that due tb distance; hence objects look larger, for the eye jtfdges of the size of an object by multiplying the size of an image or impression received by the square of the distance, while the latter is estimated from the indistinctness of the object. In the fog the apparent distance is increased, but the eye interprets it as clue to the opposite cause. On looking at the photograph of a tree, a church, a monument, or a pyramid, it is not possible to form a correct idea of its size unless a man or animal ’ig seen in the same view, with which to campare it. In nature, especially on land, the intervening objects that lead up to it give the data on which to calculate the distance. Where none intervene, as in looking from peak to peak, the eye must depend on distinctness, and when the air is verv clear and transparent, as in Colorado, distances seem less than they are. If the object is seen through transparent, but cold, media, the form remains true, but the colors are changed.— Scientific American.