246 Mr. Shaw on the f Sense"* of Muscular Action 



become convinced that the body whose image is in the eye 

 exists externally. To 'trace' or 'see along' aline, as the 

 expression is used, includes the opinion that the object is 

 placed externally ; it likewise includes the idea of guiding or 

 directing the eye ; and, in addition to these, it implies the ex- 

 istence of a coloured image painted upon the retina. The 

 notion conveyed by these words is, therefore, most compli- 

 cated ; and they ought never to have been applied, as has been 

 done, in speaking of the functions of a single nerve. 



Another inconsistency in the theory may be noticed. The 

 theory rests upon the supposition that the retina can distin- 

 guish the direction of the object by seeing along lines which 

 lead from the image to the external body. But it is incom- 

 prehensible how the nerve, which is seated at the bottom of 

 the eye, should be able to ascertain, by itself, the direction of 

 rays or lines which terminate in it. These rays of light are 

 only sensible when they arrive at that point in the surface of 

 the retina which is their final destination ; and they are not 

 recognizable by any other nerve of sense besides the optic 

 nerve. Now, according to the most elementary definitions in 

 mathematics, when we desire to learn the direction of a line, 

 it is necessary that two points in it, at least, should be made 

 known ; but, according to the theory, it is the single point 

 alone which terminates the ' line of visible direction ' that is 

 made sensible. If we could conceive that the line, in passing 

 through the anterior surface of the eye, caused a sensation of 

 the particular spot in the cornea where it entered, we might 

 then have two points presented to the mind, by which to esti- 

 mate the direction of its passage from the object to the retina: 

 but, without such a second point, it appears quite impossible 

 to ascertain its direction. 



The question as to the manner in which the idea of the 

 'direction' of objects is obtained, is to be approached in 

 a very different way than by attending to the rays which 

 proceed from them : it requires a more complex operation of 

 the senses to acquire this knowledge of surrounding objects 

 than has been conceived. We have already seen that, in order 

 to ascertain the simple truth that an object is situated exter- 

 nally, something more is necessary than a mere impression 



