VOL. LXVIII.] PHILOSOPHICAL TRANSACTIONS. 301 



the vitreous humour, like a thin white worm. The use of this may be to supply 

 the vitreous humour or crystalline with nourishment, whether it be a nerve or 

 an empty blood-vessel ; but this is certain, that its rising so high above the sur- 

 face of the retina must render it incapable of vision : whence there is just reason 

 to conclude, that this conformation must be the true cause of the insensibility 

 of this part of the eye. 



Dr. D. does not affirm, that the human eye, either during infancy or in our 

 riper years, is similar in conformation to that of a calf, nor have we (he remarks) 

 sufficient opportunities to observe them ; but he suspects this vessel may, after 

 the growth of the animal, be totally obliterated ; and that, in some few in- 

 stances, the optic nerve may even in this part become sensible to light. One 

 instance he was certain he had seen, as it was in a man capable of the most pa- 

 tient and accurate observation, who on numberless repeated trials, at different 

 times, in his presence, could never lose sight of the smallest object with either 

 of his eyes. 



Supplement to the Case in Squinting. — It afterwards occurred to Dr. D. that 

 the unusual mode of squinting described in the above paper, must have arisen 

 from some original difference in the sensibility of some parts of the eye, which 

 might have rendered it more easy for master Sandford, when a child, to observe 

 objects with one eye only, and that with the eye most distant from objects pre- 

 sented obliquely to him. 



Two circular papers, each of 4 inches diameter, were stuck against the wall, 

 their centres being exactly at 8 inches distance from each other, On closing one 

 eye, and viewing the central spot of one of these papers with the eye farthest 

 from it, and then retreating 26 inches from it, the other paper became invisible. 

 This experiment was made on 5 people, of various ages, from 10 years old to 

 40 ; and the paper disappeared to them all at about this distance, or an inch or 1 

 more or less : but to master Sandford the paper disappeared at about 13 inches 

 distance from the wall. These papers were afterwards removed to 12 inches, 

 and then to 4 inches interval between them ; and by the nicest observations on 

 repeated trials he found, that the paper, equally with one eye as with the other, 

 uniformly disappeared to him at about 4- the distance it did to 5 others. 



Another curious circumstance is, that as large a paper disappeared to him at 

 ithe distance as it did to others at the whole distance ; and hence the insensible 

 part of the centre of the optic nerve in his eyes was, as near as could be estimated, 

 4 times the area of the insensible part of the eyes of other people, at the same 

 time that the angle made between the ingress of the optic nerve and the bottom 

 of the eye was twice as great as in others. 



It is easy to conceive that, in early infancy, when any object which the child 



