244 Mr. Shaw on the ' Sense ' of Muscular Action 



If the experiment with the card be made as he has directed, 

 it will be found to be true that the image F retains the same 

 position, whichever ray, proceeding from the object, is per- 

 mitted to enter the eye ; and it appears in the same place, 

 whether all the rays, or only one be admitted. But this by no 

 means leads to the conclusion that the direction of the object 

 is ascertained by the retina pursuing the object along any par- 

 ticular line. It shows only, that the same sensation is excited 

 whether a ray falls obliquely upon the retina, either from above 

 or from below, or falls upon it in a perpendicular line ; and 

 that, therefore, no contrast can be made between them. It 

 ought rather to have been noticed that all the rays from the 

 external object concentrate towards a single point in the retina 

 F that each individual ray, however separated from the others 

 in its course, must proceed from the same point in the object, 

 and affect the same spot in the retina that, consequently, 

 no difference in the sensation is to be expected and that it 

 is altogether futile to look upon the direction of the several 

 rays as leading to any knowledge of the place of the object. 



It is not easy to understand what is meant by the expression 

 1 seeing along ' lines, when these lines stretch outwardly from 

 the retina through the humours of the eye and through the 

 atmosphere. Dr. Porterfield has said that ( by virtue of a 

 connate and immutable law, the mind traces back its own sen- 

 sation, and sees every point of the object, not in the sensorium 

 or retina, but without the eye, in those perpendicular lines,' 

 which Dr. Brewster has called the lines of visible direction.' 

 This is supposing us to possess a power which reaches far 

 beyond where the optic nerve is present to exercise it. It is 

 also presuming that the mind has a consciousness of the out- 

 ward object, distinct from that sensation which is conveyed by 

 the image formed upon the nerve. 



But it may be asked, how is the consciousness of the object 

 being external obtained ? how do we acquire the knowledge 

 of the simple fact, that the body, whose image is painted in the 

 interior of our eye, is exterior to us ? It is not through the 

 medium of the optic nerve alone that this information is ob- 

 tained. Our sole knowledge of the existence of an external 

 body, so far as the nopticerve is concerned, is acquired through 



