212 EECEXT PROGRESS OF THE THEORY OP VISION. 



excites a separate impression ; that the difference of the 

 several points of the field of vision in degree of brightness 

 can be appreciated by the sense ; and lastly, that separate 

 impressions may each arrive separately at the seat of 

 consciousness. 



If now we compare the eye with other optical instru- 

 ments, we observe the advantage it has over them in its 

 very large field of vision. This for each eye separately is 

 160° (nearly two right angles) laterally, and 120° verti- 

 cally, and for both together somewhat more than two 

 right angles from right to left. The field of view of in- 



FiG. 30. 



struments made by art is usually very small, and becomes 

 smaller with the increased size of the image. 



But we must also admit, that we are accustomed to 

 expect in these instruments complete precision of the 

 image in its entire extent, while it is only necessary for 

 the image on the retina to be exact over a very small 

 surface, namely, that of the yellow spot. The diameter 

 of the central pit corresponds in the field of vision to an 

 angular magnitude which can be covered by the nail of 

 one's forefinger when the hand is stretched out as far as 

 possible. In this small part of the field our power of 

 vision is so accurate that it can distinguish the distance 

 between two points, of only one minute angular magni- 

 tude, i.e. a distance equal to the sixtieth part of the 

 diameter of the finger-nail. This distance corresponds to 



