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 tliis 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 



