THE EYE. 



Fig 13. 



e 7 ;ey /o 



L II 



of the line L E. This wand will be seen by the left eye in the direction 

 of the division 8, and by the right eye in the direction of the division 



4, on the screen, and two images 

 o the wand will accordingly be 

 - perceived ; but, according as the 

 attention is directed to the one or 

 to the other, a consciousness of 

 them will be produced. Thus, 

 by an act of the will we may con- 

 template only the objects as seen 

 with the left eye, in which caso 

 the wand will be seen projected 

 on the screen perpendicular to 

 the line A B, at the 8th division ; 

 and by a like act of the will, the 

 attention being directed to the 

 impression produced by the right 

 eye, the wand will be seen pro- 

 jected on the screen at the 4th 

 division of the scale. If the at- 

 tention be withdrawn from either 

 of these and the wand be viewed indifferently, we shall be conscious 

 of the two images, but not with the same distinctness as that with 

 which we should perceive two wands placed at the 4th and 8th 

 divisions of the scale. It will follow from this, that when we look 

 with both eyes at any object, such as the printed page of a book, 

 at the distance of 8 or 10 inches from the eyes, we have two 

 images of the different parts of the page placed before the eyes, 

 which are seen in different directions, and ought therefore to pro- 

 duce double vision ; but this is prevented by habitually directing 

 our attention to one of the two, and neglecting the other. 



That the perception of an object will be double if the directions 

 in which it is seen by the two eyes are different, may also be 

 demonstrated in the following manner : 



It has been already shown that the optical centres of the eyes, 

 cannot change their position by the mere action of the muscles 

 which move the eye-balls in their sockets, and that the direction 

 in which any distant object is seen by both eyes is the same, and 

 hence it is perceived single ; but if a slight pressure be applied to 

 the eye with the finger, the optical centre of the eye may bo 

 moved from its position, so that the direction of the same object 

 seen by it and the other eye will not be the same. A distant 

 object will in this case be seen double, being perceived in ono 

 direction by the eye which retains its natural position, and in 

 another by that whose position is deranged by pressure. 

 86 



