THE EYE. 



Thus, for example, if the eye be at E (fig. 14), any object which 

 moves from A to B will cause the line of direction in which it is 

 seen to revolve through the angle A E E, just as though the body 

 which moves were to describe a circular arc, of which E is the 

 centre and E A the radius. But if, instead of moving 'from A to it, 

 the body were to move from A' to E', the impression which its 



motion would produce 

 upon the sight would 

 be exactly the same. 

 It would still appear 

 to be moving from the 



direction E A' A to ths 



" " n . .. 



direction E B B . 



In fine, the eye affording no perception of direct distance, 

 supplies no evidence of the extent to which the body may change 

 its distance from the eye during its motion, and the apparent 

 motion will be the same as if the body in motion described a 

 circle of which the eye is the centre. 



Hence it is that the only motion of which the eye forms any 

 immediate apprehension is angular motion, that is, a motion 

 which is measured by the angle which a line describes, one 

 extremity of which is at the centre of the eye, and the other at 

 the moving object. 



85. Though the real direction in which a distant object moves 

 cannot be obtained by the direct perception of vision, some 

 estimate of it may be formed by comparing the apparent angular 

 motion of the object with its apparent magnitude. 



Thus, for example, if we observe that the apparent magnitude 

 of an object remains constantly the same while it has a certain 

 apparent angular motion, we infer that its distance must neces- 

 sarily remain the same, and consequently that it revolves in a 

 circle, in the centre of which the observer is placed ; or if we find 

 that it has an angular motion, in virtue of which it changes its 

 direction successively around us, so as to make a complete circuit of 

 360, and that in making this circuit its apparent magnitude first 

 diminishes to a certain limit, and then augments until it attains a 

 certain major limit from which it again diminishes, we infer that 

 such a body revolves round us at a varying distance, its 

 distance being greatest when the apparent magnitude is least, 

 and least when its apparent magnitude is greatest. An exact 

 observation of the variation of the apparent magnitude would in 

 such a case supply a corresponding estimate of the variation of the 

 real distance, and would thus form the means of ascertaining the 

 path in which the body moves. 



86. An example of this is presented in the cases of the sun and' 

 90 



