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TEXT-BOOK OF PHYSIOLOGY. 



distance of two meters the vertical horopter is a plane. Within this 

 distance it is concave to the face ; beyond this distance it is convex. 



An object which lies either in front of or behind the fixation point 

 will project its rays on parts of the retinae which do not correspond, 

 and hence give rise to double images. This is evident from examina- 

 tion of Fig. 306. While the eyes are directed to figure 2, of which 

 there is but a single image, the objects B and A give rise to double 

 images, for reasons already given. If the eyes are now directed to 

 B, double images will be formed of 2 and A. 



At all times, therefore, double images are formed on the retinae 

 the existence of which is scarcely noticed unless the attention is 



FIG. 305. HOROPTER FOR THE 

 SECONDARY POSITION, WITH 

 CONVERGENCE OF THE VIS- 

 UAL AXES. (Landois.) 



FIG. 306. SCHEME OF IDENTICAL AND NON- 

 IDENTICAL POINTS OF THE RETINA. 

 {Landois.) 



directed to them. This is due to the fact that many of the images 

 fall on the peripheral, less sensitive parts of the retinae. At the 

 same time, from a want of accommodation and the formation of 

 diffusion- circles, they are indistinct. For these reasons they are 

 readily neglected. 



In the primary position of the eyes that is, a position in which 

 the visual axes are parallel the horopter is a plane in infinity. In 

 the tertiary position the horopter is a curve of complex form. 



Movements of the Eyeball. The almost spheric eyeball lies 

 in the correspondingly shaped cavity of the orbit, like a ball placed 

 in a socket, and is capable of being moved to a considerable extent 



