VISION. 



803 



nea a little to the inner side of its center, and therefore forms an 

 angle with the optic axis, which is termed the angle alpha, which 

 normally does not exceed 4 to 5 degrees. 



HOROPTER. The horopter represents all those points of the 

 outer world from which rays of light passing into both eyes fall on 

 identical points of the retina, the eyes being in a certain position. 



It is a circle of which the chord is formed by the distance 

 between the point of decussation of the rays of light in the eye. Its 



B 



F O 



Fig. 353. The Horopteric Circle of Muller. (BALL.) 



size is determined by the position of the two eyes and the point 

 towards which their axes converge. All objects not found in the 

 horopter, or which do not form an image on corresponding points of 

 the retina, are seen double. 



INVERTED IMAGE OF OBJECTS. The rays proceeding from the 

 surface of luminous bodies above the optic axis cross in the eye so 

 as to be brought to a focus below the axis, and vice versa. Thus an 

 inverted image is formed on the retina. 



THE ACUTENESS OF VISION is tested by Snellen's types. It has 

 been found out that square letters which have limbs and parts equal 



