PART III. 



Of some Consequences from tfie foregoing Theory of 

 Objects being 1 seen single with two Eyes, together with 

 the Explanation of several other Phenomena of Vision, 



IT has hitherto, I believe, been thought by 

 opticians, that, if the position of the eye be un- 

 changed, the visible direction of an object will be 

 the same, as long as its picture occupies any one 

 point of the retina ; and that, in every different 

 position of the eye, a picture, which continues to 

 occupy the same point of the retina, will repre- 

 sent its object in a different direction. But if 

 the theory be just, which I have advanced in 

 the preceding part of this Essay, neither of 

 those opinions can be universally true. For it 

 follows, from what was there mentioned, that 

 if one of the optic axes be kept fixed, and the 

 other be at different time's variously bent to- 

 ward it, objects, though situated in the fixed 

 axis, will nevertheless change their visible di- 

 rections, with every variation of the moveable 

 axis j since they must always appear in the 

 common axis, which alters its position with 

 every change of the moveable axis : And again, 

 that>, if the two optic axes should vary their 



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