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Page:Popular Science Monthly Volume 89.djvu/448

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434

��Popiilar Science Monihly

���APPAR LOWER

��Spacing of Strings Will Appear Different to Each Eye When Head Is Tilted

The Reason Why We Have Two Eyes Spaced Apart

IN the March issue of Popular Science Monthly there appeared an article explaining why we have two eyes which showed how the possession of two eyes gives us stereoscopic vision. To this might be added the explanation of another important i)r()\ision of nature growing out of it, namely, why the eyes are spaced apart on a horizontal line instead of one above the other. If we look at several strings or wires spaced apart and running horizontally across the line of vision as, for examjile, a group of telephone or telegrai)h wires stretched horizontally, they will appear to lie in tile same i)lane and all the same distance from the eyes, but upon tilling the head to one side so as to bring the eyes one over the other, the wires will appear in their true relation to each other and to the eyes.

With two eyes our stereoscopic vision is limited to the dimensions running parallel to the plane of the eyes; in cither words, till- horizontal dimensions of ol)- jects and the horizontal spacing iictwci ii them. As this limitation is unavoi<i.il>lc, the advantage of having the eyes spaced apart in a horizontal [)lane, so that the stereoscopic elTecl will reveal the hori-

��zontal relation of objects, will be ap- parent when it is noted that most things in nature, lying in the path of tra\el, are horizontally spaced apart, such for ex- ami)le as trees, men and beasts walking on the same level and the vertical edges of taller objects. We are seldom called upon to avoid objects by passing over or under them.

To this line of thought might also be added the conclusion that perfect stereo- scopic vision in all planes is attained through a third eye positioned above and equidistant from the first two, but this, being of little necessity to our safety, nature has not provided, and its omission can be somewhat compensated for by tilting the head to one side, a thing which we often see people do.

A simjjle way to verify this is to tie a few strings to the legs of a chair as in- tlicated in the above sketch, with some strings to the front and some to the back of the legs and the points of contact hidden from view. The diagram explains how the spacing of the strings will appear diffi-rently to each eye when the head is tilted to bring one eye above the other, giving a stereoscopic effect, but with botii eyes on the same level, as for ex- ample the upper one, the spacing will appear the same to both.

Belt Shifter Protects Workmen

FOR preventing some of the serious accidents that are caused b>' work- men shifting belts rapidh', a Bridgeport company has brought out a belt shifter operating on a new principle.

Tai)ered rollers project from the end of a pole. The loosened belt is caught between these rollers and forced back on the pulley. By having the rollers tapiTcd, the belt auto- matically works towarfl their bases. This fiMlure |)revents the belt Hying on t w a rd s and possibh' t'ntang- ling the work- man, with dis- astrous results.

���The Loosen- ed Belt Is Caught Be- tween Roll- ers and Set Bark on the Pulley

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