SIGHT AND THE VISUAL ORGAN. 471 



particular spot, situated somewhat beyond the optic nerve and exactly 

 opposite the centre of the cornea. The light which falls along the 

 main axis of the eye converges at this point. This spot in the 

 retina is marked by a small hollow. For the rest, it is filled up with a 

 structure of its own ; and we have reason for assuming that it fur- 

 nishes the most exact perceptions, not only on account of the greater 

 optical sharpness of the image, but also on account of the higher energy 

 or activity with which it is endowed. It is this spot we make use of 

 when we desire to go into details ; for, if we wish to examine closely 

 into the nature of an object, either we approach it to the eye, or bring 

 the eye to bear on the object ; but, in both cases, in such a manner as 

 to cause the image to fall exactly on the hollow of the retina, or on 

 the spot of direct vision. This arranging of a position for an object is 

 what we mean when we speak of adjusting the eye. 



The images which are not projected on the spot of direct vision are 

 not sharp ; for the necessary light falls on the refracting media more 

 or less obliquely. This, and the decrease in the activity of the sight 

 from the hollow to the sides, explain how the objects, the farther they 

 are removed from the fixed point, appear with so much less clearness 

 and sharpness of outline. Indirect or eccentric vision, as it is termed, 

 makes us aware simply of the presence of objects, by giving us some 

 notion of their shapes ; but we are unable to distinguish even the big- 

 gest letters, if the image of them should fall only one hair's breadth 

 off that one spot in the retina. In reading, the eye must constantly 

 move onward to the end of the line, the single letters thus gradually 

 imprinting themselves on the direct point of vision. On the other 

 hand, indirect vision offers hints for fixing the object in our eye ; it 

 wai-ns us of, and prepares us for, the object previous to our devoting 

 our whole attention to it, and it is further of use in procuring a wide 

 survey, by enabling us to see and examine what lies before us. There 

 are some who only possess direct vision. Any one can put himself in 

 the place of an individual so afflicted, by holding a long tube of small 

 calibre to his eye. You naturally distinguish the most minute objects 

 enclosed within the restricted range of vision ; but, deprived of th& 

 lateral impressions, you could not guide your steps in the street. In 

 short, you must fancy the image of the external world that is imprinted 

 on the retina, like a picture highly finished in the middle, and only 

 roughly sketched out at the sides. 



The distance from the spot of direct vision, at which the objects 

 may be perceived by eccentric vision, has its limits. When looking 

 straight before you, you can just perceive a hand which stretches down 

 the whole length of the face, in the direction of the temples. This is 

 the extremest point from which it is possible for light to fall on the 

 retina; but, if you attempt to go beyond it, the hand disappears, its 

 image not being projected on the retina. The combination of all the 

 extreme points from which, with a set eye, impressions may be received, 



