64 



SPECIAL SENSES. 



[ 133. The motions of the eye are of great importance in 

 the act of vision. As in the steady contemplation of objects 

 we have to bring them into the focal centre of the produced 

 visual axis, we necessarily move the eye-ball in the act of 

 looking around and studying the details of objects successively, 

 according to determinate laws. It has been ascertained that 

 in this motion the eye-ball revolves accurately round a point, 

 the point of revolution of the eye which remains unaltered ; 

 it is at once the point of intersection of the rays of direction 



Fig. 4 1 



/' 



and of those of vision. 

 In this point (fig. 41), 

 a, in the appended di- 

 agram, all the diame- 

 ters of the eye inter- 

 sect, and many of 

 these diameters are at 

 the same time the 

 axes of revolution 

 with reference to the 

 actions of the muscles 

 of the eye. If the 

 two eyes be directed 

 to the points b and b\ 

 dthe axal rays fall upon 

 c and c'. Both eyes 

 then look forwards, 

 and also somewhat 

 convergingly, so that 

 the two axes b c, and 

 ' c', do not run pre- 

 cisely parallel, but diverge slightly, by which c and c' are 

 further from one another than b and b'. In the horizontal 



ground. From the diagram it is seen that in the right eye, the spot, a, falls 

 upon the point of the retina, a', whilst the cross, b, falls in the middle of the 

 entrance point of the optic nerve, precisely where the central artery and 

 vein of the retina are situated. Now if the left eye be closed, and the 

 point a, and cross b, are regarded at the usual distance for distinct vision, 

 the attention being, however, particularly directed to a, the cross #, 

 will be found to disappear the moment the pencil of rays proceeding 

 from it comes to fall upon the middle of the entrance place of the optic 

 nerve. 



