188 RECENT PROGRESS OF THE THEORY OF VISION. 



instead of the piece of paper, the retina. Accordingly, if an 

 optically accurate image is thrown upon the retina, each of its 

 cones will be reached by exactly so much light as proceeds from 

 the corresponding point in the field of vision; and also the 

 nerve fibre which arises from each cone will be excited only by 

 the light proceeding from the corresponding point in the field, 

 while other nerve fibres will be excited by the light proceeding 

 from other points of the field. Fig. 30 illustrates this effect. 

 The rays which come from the point A in the object of vision 

 are so broken that they all unite at a on the retina, while those 

 from B unite at b. Thus it results that the light of each separate 

 bright point of the field of 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, 



FIG. 30. 



that separate impressions may each arrive separately at the seat 

 of consciousness. 



If now we compare the eye with other optical instruments, 

 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 vertically, and for both together 

 somewhat more than two right angles from right to left. The 

 field of view of instruments made by art is usually very small, 

 and becomes smaller with the increased size of the image. 



But \ve 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 



