6l4 Philosophical transactions. [anno i683. 



both eyes in that position, but on the internal of one, suppose the left eye, and 

 the external lateral of the right ; which would cause a double perception. This 

 objection I foresaw when I hinted that whether the nerves decussated or not, it 

 would be no prejudice to my opinion, nay perhaps might more fully confirm 

 my opinion where they do. In that passage I had respect also to the inversion 

 of the image in the eye being rectified in the brain ; though that equally presses 

 any other hypothesis, and the explication of the thing may be well enough 

 understood by a blind man's judging of the position of an object above his head 

 by touching it with one end of his stick, though the other end terminates under 

 that object or in his hand : and so in our view of an object, the true situation 

 of the respective parts is not distinguished so much by means of that end of 

 the ray that terminates in the eye, as of the other end that touches the object, 

 from whence the vibration or protrusion comes. 



But to leave this, which does not so particularly concern me, I come to the 

 objection itself, as it relates to my opinion ; and though it seems at first view 

 the most difficult of all to be answered, yet it may be determined by the experi- 

 ment itself, better than by the scheme (in the Philos. Collections), where the 

 eyes are not drawn in that position that is here required. Now let there be 

 placed an object near the left eye of any person, but not so near that eye as that 

 the nose might hinder the rays from falling on the right, because it is to be 

 seen with both, and whilst that person looks on it let a by-stander obwserve the 

 position of both eyes, and he shall see that the pupil of the right eye is turned 

 in a very oblique manner to the object, whereas the pupil of the left is scarcely 

 so at all, whereby there will be three parts to one more in the distance of the 

 pupil of the right eye from the external canthus, than there will be in the other. 

 Hereby it appears that if the object be so placed that it is seen with both eyes, 

 the right eye accommodates itself to the position of the left, that the rays strike 

 correspondent fibres, and the percussion or vibration being toward the bottom 

 or papilla of the eye, a small turning of one eye to another will make that ac- 

 commodation. 



Moreover as this accommodation is made in an oblique position of the object, 

 so is it more readily done in a direct position of the same; and this we may per- 

 ceive in an object's retiring in a straight line from the eyes, whereby the pupils 

 gradually divaricate; as on the contrary they converge when the object is seen 

 very near them, and that so forcibly that it is painful to hold them long in that 

 posture. 



4. But to proceed to Other objections: the case of cross-eyed persons by birth 

 I have considered at the end of the forementioned essay, and shown why a 



