146 Mr Twining on Single Vision, 



optic axes are parallel, or meet in a point, the two middle 

 points of the retinas, or any points which are equally distant 

 from them, and lie on the same sides of them, either towards 

 the right hand, or left hand, or upwards or downwards, or in 

 any oblique direction, are called corresponding points.'" 



Wells objects to this, which, he observes, attributes the 

 joint possession of one property, to places of the retinas at 

 unequal distances from the centres of the optic nerves ; and 

 that a point of the retina which is external in one eye, has a 

 correspondence of action and sensibility, with a point of the 

 retina in the other eye, which is internal. For instance, if we 

 •look at an object with both eyes, which is removed some dis- 

 tance, say 25 degrees, either to the right or left of the place 

 immediately in front of us, the pictures fall on that part of the 

 retina lining the temporal side of one eye, and the nasal side of 

 the other eye. Wells's theory, derived from consideration of 

 visible direction and visible distance, does not appear to explain 

 the difficulty. 



The theory of single vision which Dr Wollaston has lately 

 deduced, from the assumed semidecussation of the optic nerves, 

 and his reasoning founded on cases of disease in man, together 

 with the facts which comparative anatomy afford, completely 

 meet the objection which Dr Wells has urged against the theory 

 of corresponding points of the retinas above noticed : assign- 

 ing, at the same time, a very plausible reason for that corre- 

 spondence in the anatomical structure, arising from the semi- 

 decussation which he believes to exist. 



Now the structure which Dr Wollaston imagines to exist 

 in man, namely, the semidecussation of the optic nerves, and 

 the formation of retina thence supposed to arise ; so that the 

 nasal side of the retina in each eye is formed by fibres aris- 

 ing from one thalamus ; and the temporal side of the retina 

 by fibres from the other thalamus ; if admitted in the fullest 

 extent that he requires, only accounts for double vision when 

 there is disaccordance of the optic axes in a horizontal direc- 

 tion. But we find that non-accordance of the optic axes in a 

 vertical direction will also produce double vision, which is not 

 accounted for by the structure which Dr Wollaston imagines. 

 Moreover, I think the cases and dissections presently to be 



