THE SENSE OF SIGHT. 



671 



is also an horopter for the vertical line of separation. At a 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 examination of Fig. 333. 

 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. 

 It 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 directed to them. 



FIG. 332. HOROPTER FOR THE 

 SECONDARY POSITION, WITH CON- 

 VERGENCE OF THE VISUAL AXES. 

 (Landois.) 



FIG. 333. SCHEME OF IDENTICAL AND 

 NON-IDENTICAL POINTS OF THE RETINA. 

 (Landois.) 



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 accommo- 

 dation 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 at infinity. In the tertiary 

 positions 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 rotated to a considerable extent by the six muscles 

 which are attached to it. These muscles are the superior and inferior recti, 

 the external and internal recti, and the superior and inferior obliqui (Fig. 

 334). The four recti muscles arise from the apex of the orbit cavity, from 

 which point they pass forward to be inserted into the sclera about 7 to 8 

 mm. from the corneal border. The superior oblique muscle having a similar 

 origin passes forward to the upper and inner angle of the orbit cavity, at 



