280 Mifcellanea Curiafa. 



Again ; in our two Eyes (when the ObjeA 

 k feen by both) there is yet another means 

 of. eftimating how far off it is. (And it is 

 this by which we judge of Diftances.) Name- 

 ly, there are, from the fame Object, two dif- 

 ferent vifual Cones, terminated at the two 

 Eyes : Whofe two Axes contain, at the Ob- 

 ject, different Angles, according to different 

 Diftances : An accuter Angle at a great 

 Diftance, and more obtufe when nearer. 



Now, that fuch Object may be feen by both 

 Eyes, clearly \ it is requifite that the Eyes be 

 put into fuch a Pofition, as that the Sight of 

 each Eye receive the refpe&ive Axe at right 

 Angles. Which requires a different Pofition 

 of the two Eyes, according to the different 

 Diftance of the Object. 



As will manifeftly appear \ if we look, wi th 

 attention, on a Finger (or other fmall Ob- 

 ject) at two or three Inches diftance from the, 

 Eye ; and then upon another like Object at 

 three or four Yards beyond it : (and this al- 

 ternately feveral times. For 'twill be mani- 

 feft, that while we look intently on the one, 

 we do not fee the other (or but confufedly) 

 though both be juft before us. And, as we 

 change our view, from the one to the other, 

 we manifeftly feel a Motion of the Eyes (by 

 their Mufcles) from one pofture to ano- 

 ther. 



And according to the different pofture in 

 the Eyes, requilite to a clear Vifion by both, 

 we eftimate the Diftance of the Object from 



flS- 



And hence it is,that they who have loft the 

 Sight of one Eye, are at 3 great difadyantage, 



m 



