1824.] Philoiophical Transaction for 1823, Part II. 63 



involuntary muscles draw the pupil under the upper eye-lid. 

 This is the condition of the organ during perfect repose. 



" On the other hand, there is an inseparable connexion be- 

 tween the exercise of the sense of vision and the exercise of the 

 voluntary muscles of the eye. When an object is seen, we 

 enjoy two senses : there is an impression upon the retina ; but 

 we receive also the idea of position or relation which it is not the 

 office of the retina to give. It is by the consciousness of the 

 degree of effort put upon the voluntary muscles, that we know 

 the relative position of an object to ourselves. The relation 

 existing between the office of the retina and of the voluntary 

 muscles, may be illustrated in this manner. 



" Let the eyes be fixed upon an illuminated object until the 

 retina be fatigued, and in some measure exhausted by the 

 image, then closing the eyes, the figure of the object will con- 

 tinue present to them : and it is quite clear that nothing can 

 change the place of this impression on the retina. But notwith- 

 standing that the impression on the retina cannot be changed, 

 the idea thence arising may. For by an exertion of the volun- 

 tary muscles of the eye-bail, the body seen will appear to change 

 its place, and it will, to our feeling, assume different positions 

 according to the muscle which is exercised. If we raise the 

 pupil, we shall see the body elevated, or if we depress the pupil, 

 we shall see the body placed below us ; and all this takes place 

 while the eye-lids are shut, and when no new impression is con- 

 veyed to the retina. The state of the retina is here associated 

 with a consciousness of muscular exertion ; and it shows that 

 vision in its extended sense is a compound operation, the idea 

 of position of an object having relation to the activity of the 

 muscles. 



" We may also show, by varying this experiment, that an 

 agitated state of the muscles, or a state of action where the 

 muscles are at variance or confused, affects the idea of the 

 image. If we look on the luminous body so as to make this 

 impression on the retina, and then cover the face so as to exclude 

 the light, keeping the eye-lids open, and if we now squint, or 

 distort the eyes, the image which was vividly impressed upon 

 the retina instantly disappears as if it were wiped out. Does 

 not this circumstance take place, because the condition of the 

 muscles thus unnaturally produced, being incongruous with the 

 exercise of the retina, disturbs its operation? 



" If we move the eye by the voluntary muscles, while this 

 impression continues on the retina, we shall have the notion of 

 place or relation raised in the mind ; but if the motion of the 

 eye-ball be produced by any other cause, by the involuntary 

 muscles, or by pressure from without, we shall have no corre- 

 sponding change of sensation. 



"If we make the impression on the retina in the manner 

 described, and shut the eyes, the image will not be elevated, 



