SINGLE VISION. 67 



only necessary to determine, whether the de- 

 pendance of visible direction upon the actions 

 of the muscles of the eyes be established by 

 nature, or by custom. But facts are here 

 wanting. As far as they go, however, they 

 serve to prove, that it arises from an original 

 principle of our constitution. For Mr. Chesel- 

 den's patient saw objects single, and conse- 

 quently in the same directions with both eyes, 

 immediately after he was couched; and persons 

 affected with squinting from their earliest in- 

 fancy, see objects in the same directions with 

 the eye they have never been accustomed to em- 

 ploy, as they do with the other they have con- 

 stantly used. 



Having thus shown in what directions ex- 

 ternal bodies are seen, when their situation 

 with respect to the eye is given, and upon what 

 circumstance the various directions depend, in 

 which a picture upon any one place of the 

 retina can exhibit the object producing it ; I 

 should render the theory of visible direction 

 complete, were I now to point out the relative 

 positions of the two lines of direction, in which 

 any two different parts of the retina represent 

 their objects. To ascertain this, the first step 

 must be, to find the place of the retina which 

 receives the picture 'of an object, whose situa- 

 tion with respect to the external eye is known; 



F2 



