66 THE VERTEBRATE RETINA 



paper picture is built up of large dots spaced widely, for on such rough 

 paper any finer dots would make only an inky blur. The magazine 

 photograph contains many more dots per unit area, and they are much 

 smaller. We say that the magazine picture is the better resolved of the 

 two. Similarly, we might take two photographs with the same camera 

 but using two different kinds of film whose emulsions differed greatly 

 in fineness of grain. The fine-grained picture could be enlarged much 

 more than the coarse-grained one without becoming blurry and losing 

 in detail. The fine-grained emulsion 'resolves' better what it 'sees'. 

 Again, through a well-corrected microscope lens one can see and count 

 fine dots, striations and the like which run together under less perfect 

 lenses — and again, we speak of a difference in resolving power as exist- 

 ing between the two. As we have seen, retinal images are very small; 

 but mental images are 'big as life' and the retinal image must stand 

 enormous enlargement without too much loss of detail, when it is trans- 

 lated into a mental picture of the visual field of the eye. 



The dioptric apparatus of the eye may cast upon the retina an image 

 which is relatively large or small, hazy or sharp; but the retina in turn 

 may be crudely or finely built and upon this will depend the possible 

 maximum perfection of the cerebral image. The resolving power of the 

 retina is governed by three factors, all of which vary from retina to 

 retina and the last of which may even vary physiologically from time 

 to time within a single retina: (a) the slenderness of the visual cells; 

 ib) their closeness of spacing; and (c) the number connected with one 

 optic nerve fiber. The first two of these are almost self-evident; for if 

 the images of two object-points fall upon two separate visual cells, be- 

 tween which is an unstimulated visual cell, the two object-points may 

 be resolved; but if the visual cells are so plump or so far apart that the 

 two object-points are imaged upon two adjacent visual cells, they cannot 

 be distinguished as two points and will seem the same as a single large 

 object-point whose image covers the same two adjacent visual cells. In 

 the one case, we have an analogy for the fine screen through which a 

 picture is photographed for reproduction on coated paper as a half-tone 

 electrotype; in the other case, a coarse screen like that used with news- 

 print. 



Factor "c" brings in the concept of summation presented in a pre- 

 ceding Section. Two object-points, whatever their size or separation, will 

 be seen as a single blur if their images fall upon visual cells which 

 connect with the same bipolar, or upon those whose separate bipolars 



