THE SENSES. 619' 



has an orange-colored tint upon a bright blue surface, and a bluish tint 

 upon an orange-colored surface; a yellowish color upon a bright violet, 

 and a. violet tint upon a bright yellow surface. The color excited thus, 

 as a contrast to the exciting color, being wholly independent of any rays 

 of the corresponding color acting from without upon the retina, must 

 arise as an opposite or antagonistic condition of that membrane; and the 

 opposite conditions of which the retina thus becomes the subject would 

 seem to balance each other by their reciprocal reaction. A necessary 

 condition for the production of the contrasted colors is, that the part of 

 the retina in which the new color is to be excited, shall be in a state of 

 comparative repose; hence the small object itself must be gray. A sec- 

 ond condition is, that the color of the surrounding surface shall be very 

 bright, that is, it shall contain much white light. 



Movements of the Eye. The eyeball possesses movement around 

 three axes indicated in Fig. 416, viz , an antero-posterior, a vertical, and 

 a transverse, passing through a centre of rotation a little behind the 

 centre of the optic axis. The movements are accomplished by pairs of 

 muscles. 



Direction of Movement. By what muscles accomplished. 



Inwards, Internal rectus. 



Outwards, .... External rectus. 



Superior rectus. 



Inferior oblique. 

 Downwards, , Inferior rectus. 



Superior oblique. 



Internal and superior rectus. 



Inferior oblique. 



Inwards and upwards, 

 Inwards and downwards, 

 Outwards and upwards, . . s 

 Outwards and downwards, 



OF THE SIMULTANEOUS ACTION OF THE TWO EYES. 



Although the sense of sight is exercised by two organs, yet the im- 

 pression of an object conveyed to the mind is single. Various theories 

 have been advanced to account for this phenomenon. 



By Gall it was supposed that we do not really employ both eyes 

 simultaneously in vision, but always see with only one at a time. This 

 especial employment of one eye in vision certainly occurs in persons 

 whose eyes are of very unequal focal distance, but in the majority of in- 

 dividuals both eyes are simultaneously in action, in the perception of the 

 same object; this is shown by the double images seen under certain condi- 



