in connexion with Vision. 245 



the rays which emanate from it. These rays, being reflected 

 and dispersed to a distance, according to a law of nature, from 

 all the surfaces of the body, pass into the eye, and form upon 

 the retina an exquisitely minute image or copy of the object. 

 Now it is this image alone which gives rise to the impression 

 of the object in the mind. The outward body itself does not 

 directly excite the nerve : we have to rely for the correctness 

 of our knowledge respecting it, upon the image, formed by the 

 rays, being a faithful representation of it. As the image, there- 

 fore, is at a distance from its original, and as it is perfectly dis- 

 tinct from it, how do we learn to associate it with the external 

 object ? how do we discover that it is not an ocular spec- 

 trum, or a mere phantasm, that we see ? for it is known that 

 an impression may be made upon the retina, and remain there 

 even while the object is removed altogether from our presence. 

 We can only be assured that the image represents an object 

 which is not placed within the eye, but is external to our body, 

 by calling in the assistance of, at least, one other sense : more 

 of these being brought in as evidences may strengthen our 

 conviction ; but one in addition is absolutely necessary. We 

 have the means of ascertaining the fact which we desire, in 

 the muscular apparatus that always accompanies the posses- 

 sion of an organ of vision. 



The muscles have the power of turning the eyeball either 

 towards the object or in a contrary direction. Of this we are 

 conscious. Now, it appears to be a simple conclusion to arrive 

 at that the object must have a separate existence of its own, 

 and distinct from the eye which perceives it, when, in order 

 to see the same object, we invariably find that it is necessary 

 to exercise the muscles in a particular manner. We know 

 that, if the body presented to our sight be in motion, as a bird 

 flying through the air, we must follow it with our eyes, making 

 fresh efforts to keep them in the direction of its flight, other- 

 wise it will disappear. If the image were a mere spectrum, 

 as that produced by looking at the sun, it would present itself 

 in whichever direction we happened to turn our eyes. Hence 

 it follows that, even without calling the sense of touch or any 

 of the remaining senses into operation, but depending upon the 

 knowledge acquired from merely shifting the eye about, we 



