240 Mr. Shaw on the ' Sense* of Muscular Action 



which the rays have proceeded from the object to impinge 

 upon it ; or, in other words, that the retina can receive an 

 impression, not only of the object, but of the direction in which 

 the object is presented to it. Founding upon this as a true 

 position, they find it easy to frame what they consider to be a 

 just explanation of the problem. In an inverted image, they 

 assert, the retina does not convey the impression of its parti- 

 cular parts being inverted ; but each point in the image is 

 judged to be in the direction in which the rays have proceeded 

 in falling upon it; the uppermost pencil of rays from the object, 

 falling upon the lowest part of the image, gives the sensation of 

 its proceeding from the highest part, and consequently makes 

 that part appear to be at the top, instead of at the bottom ; 

 and so, they say, it holds with regard to the lowest rays and 

 all the others. To use Dr. Brewster's words, the retina ' sees 

 along the lines of visible direction ; ' that is, the lines which 

 lead from the image in the direction of the object. 



Such is the leading proposition on which the whole theory 

 is rested. But it is surely an error to assume that the retina 

 possesses such a power as is here attributed to it. If we ask 

 What is the meaning conveyed by the words ' visible direction* ? 

 it will be seen, on reflection, that they include something 

 more than a simple sensation obtained through an organ of 

 sense. To acquire the idea conveyed by the term ' direction ' 

 alone, it is necessary that there should be a comparison ; that is 

 to say, an operation of the mind itself. We can only form the 

 idea of the particular quarter or situation in which a body is 

 placed, by informing ourselves of its position in regard to 

 another, which has been previously fixed upon as the standard 

 of our comparison. To say, then, that our knowledge of * di- 

 rection ' can be obtained at once, and can be conveyed to the 

 sensorium like an impression through the optic nerve, is to 

 employ the term in a vague and loose manner, which must 

 necessarily lead into error. 



It is contrary to all analogy to attribute to a single nerve, 

 as the optic, the possession of such incongruous powers as this 

 theory assumes. Allowing that the idea of ' direction ' could 

 be conveyed to the mind through the medium of a nerve, it 

 would follow, if this theory were correct, that the optic nerve 

 was not only sensible of the relative position of an object, but 



