THE EYE. 



socket. It appears, therefore, that when the eye-ball is moved in 

 the socket, the picture of an external object remains fixed, while 

 the retina moves under it, just as the picture thrown by a magic 

 lantern on a screen would remain fixed, however the screen itself 

 might be moved. 



Thus, if we direct the axis of the eye to the centre o, fig. 9, of 

 any object, such as A B, the image of the point o will be formed 

 at o on the retina, where the optical axis D c meets it. The axis 

 of the pencil of rays which proceed from the point o will pass 

 through the centre of the cornea D, through the axis of the crys- 

 talline, and through the centre c of the eye-ball, and the image 

 of o will be formed at o. 



!Now, if we suppose the eye to be turned a little to the left, so 



Fig. 9. 



that the optical axis will be inclined to the line o c at the angle 

 D' c o, the image of the point o will still hold the same absolute 

 position o as before ; but the point of the retina on which it was 

 previously formed will be removed to o'. The direction of the 

 point o will be the same as before ; but the point of the retina on 

 which its image will be formed will be, not at o, at the extremity 

 of the optic axis, but at o', at a distance o o' from it, which sub- 

 tends at the centre c of the eye an angle equal to that through 

 which the optical axis has been turned. 



It is evident, therefore, that although the eye in this case be 

 moved round its centre, the point is still seen in the same direc- 

 tion as before. 



But if the optical centre of the eye were different from the 

 centre of the eye-ball, the direction in which the point o would 

 be seen would be changed by a change of position of the eye. 



To render this more clear, let c, fig. 10, be the centre of the 



Fiar. 10. 



eye-ball, and c' the optical centre of the eye. Let the optical 

 axis c D, as before, be first presented to the point o of the object. 

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