62 INNERVATION. [CHAP. XVII. 



striking a mean between the two impressions is, therefore, con- 

 stantly made use of. 



The above facts abundantly prove the non-existence of absolutely 

 corresponding points on the two retinae, such as were formerly held 

 to exist. But they do not invalidate what has before been advanced 

 respecting the general correspondence of certain tracts of the two 

 retinas, and the absolute non-correspondence of others. 



Mr. Wheatstone further observes, that if two dissimilar images 

 are represented at once to the corresponding parts of the two 

 retinae, they are not blended, but seen alternately, according to 

 their distinctness and degree of illumination. This is a very sin- 

 gular circumstance, and agrees closely with what takes place when 

 dissimilar colours are viewed in the same way. 



On the subjects treated of in this chapter reference is made to the follow- 

 ing works: Zinn, Descriptio Anatomica Oculi Human! ; Haller, Elementa 

 Physiologiae, torn, v.; Porterfield on the Eye and Vision (an admirable work); 

 Dr. Jacob's paper in the Phil. Trans. 1819, and in the 12th vol. Med. Chir. 

 Trans, and the article "Eye" in the Cyclop. Anat. and Phys. by the same 

 author; Mr. Dalrymple's Anatomy of the Eye, London, 1834; the introduction 

 to Mr. Lawrence's Treatise on Diseases of the Eye, 2d edit. 1841 ; Mr. Wharton 

 Jones' Essay prefixed to Mackensie on Diseases of the Eye ; Arnold, iiber das 

 Auge des Menschen ; Scemmerring's Plates of the Eye ; Miiller's and Wagner's 

 Physiology ; Mackenzie on Vision ; Bowman (W.) Illustrations of the Anatomy 

 of the Eye in Health and Disease, now in course of publication. 



