372 PHILOSOPHICAL TRANSACTIONS. [aNNO I687. 



yet another means of estimating how far it is off; and it is this by which we 

 judge of distances, naiilely, there are, from the same object, two different visua 

 cones, terminated at the two eyes ; whose two axes contain at the object dif- 

 ferent angles, according to different distances, an acuter angle at a great dis- 

 tance, and more obtuse when nearer. Now, that such object may be seen by 

 both eyes clearly, it is requisite that the eyes be put into such a position as that 

 the sight of each eye may receive the respective axe at right angles. Which 

 requires a different position of the two eyes, according to the different distance 

 of the object. As will manifestly appear if we look with attention on a finger 

 or other small object, at 1 or 3 inches distance from the eye ; and then upon 

 another like object at 3 or 4 yards beyond it ; and this alternately several times ; 

 for it will be manifest, that while we look intently on the one, we do not see the 

 other, or but confusedly, though both be just before us. And as we change 

 our view from one to the other, we manifestly feel a motion of the eyes, by 

 their muscles, from one position to another. And according to the different 

 position in the eyes, requisite to a clear vision by both, we estimate the distance 

 of the object from us. And hence it is, that those who have lost the sight of 

 one eye, are at a great disadvantage as to estimating distances, from what they 

 could do, while they had the use of both. 



But now when the distance grows so great as that the position of these visual 

 axes become parallel, or so near to parallel as not to be distinguishable from it ; 

 this advantage is lost, and we can thenceforth only conclude, that it is far off; 

 but not how far. Hence it is, that our view can make no distinction of the 

 moon's distance from that of the other planets, or even of the fixed stars ; but 

 they seem to us as equally remote from us, though we otherwise know their 

 distances from us to be vastly different. Because the parallax from the different 

 position of the two eyes is quite lost and undiscernible, in distances much less 

 than the least of these. And so of the fixed stars amongst themselves, which, 

 though they seem equally remote from us, may for ought we know be at dis- 

 tances vastly different. Nor can we tell which of them is nearest, unless per- 

 haps we may reasonably guess those to be nearest which seem largest. Because 

 here, not only the parallax from the distance of the two eyes, and that from 

 the earth's semidiameter, but even that from the semidiameter of the earth's 

 great orbit is quite lost, and none remaining, whereby to estimate their distance 

 from us. 



But, to return to the case in hand ; though as to small distances, we may 

 make some estimate from the known magnitude of the object ; and, as to 

 middling distances from the parallax arising from the interval of the two eyes ; 

 yet even this latter will hardly reach beyond, if so far as, the visible horizon : 



