CHAP. XVII.] CORRECT VISION WITH AN INVERTED IMAGE. 57 



behind the exact focus. It is obvious that a linear object would 

 be more perceptible than a point, and a moving object more so than 

 a stationary one, in consequence of wider and more distant portions 

 of the retina being affected in both cases. 



The apparent truthfulness of a view, recently put forward on 

 high authority in Germany, and copied into several works in this 

 country, makes it necessary to explain here that the rod-like par- 

 ticles of Jacob's membrane, though corresponding nearly in size 

 with the points of the retina capable of independent sensation, yet 

 being on the choroidal surface, and separated from the gray nervous 

 layer by the intervening granules, can scarcely have a share in 

 determining the size of the independent visual points. The un- 

 fortunate error which placed these rods as papillae on the hyaloid 

 surface of the retina, was too tempting a ground of theory not to 

 be readily admitted as true, without scrupulous examination ; and 

 the price to be paid will probably be some degree of discredit 

 thrown on minute anatomical research. 



Correct Vision with an Inverted Image. Visual Idea of Direc- 

 tion. The image on the retina being the reverse of the picture of 

 external objects seen by the mind, it is manifest that in some way 

 or other the inversion is counteracted ere the impression becomes 

 a sensation. It is conceivable that this correction may take place 

 in the optic nerve or brain, but it is far more probable that it 

 occurs in the retina. It is certain that we do not see the image as 

 it exists on the retina, or its inversion would not have remained so 

 long unknown; we rather see out of or from the retina. The 

 simple experiment of pressing with the finger on the retina through 

 the ocular tunics, and thus eliciting a luminous appearance on the 

 opposite side, seems to prove that the apparent projection of a 

 luminosity in a direction perpendicular to the point stimulated, is a 

 necessary part of the excitability of the retina. If this be granted 

 as an ultimate fact, it will explain why an inverted image, formed 

 on a concave retina, shows objects in the same position as they are 

 shown by the other senses which receive direct impressions from 

 them, particularly touch. 



It has been supposed by Miiller and Volkmann, that objects do 

 really appear inverted ; but they argue that, as long as all do so, 

 even visible parts of our own bodies, there is no need of a correc- 

 tion. But this will not explain the perfect harmony existing 

 between impressions conveyed through the senses of hearing and 

 touch, with those derived from sight. Sounds are appreciated, and 

 tactile impressions are felt, as proceeding from a particular direction 



