VISION. 543 



cerned, especially of the cortical region, where the cell processes 

 involved in vision are finally completed. 



Color- Visiou, — As we are aware by experience the range and 

 accuracy of color perception in man is very great, though vari- 

 able for different persons, a good deal being dependent on culti- 

 vation. However, there are also pronounced natural differ- 

 ences, some individuals being unable to differentiate between 

 certain primary colors as red and green, and so are "color- 

 blind." It is of course difficult to determine in how far the 

 lower animals can discriminate between colors ; but in certain 

 groups, as the birds, it would seem to be reasonable to conclude 

 that their color-perceptions are highly developed. 



It is further probable that in this group, and possibly some 

 others with the eyes placed more in the lateral than the anterior 

 portion of the head, the retinal area for the most distinct vision, 

 including that for colors is larger than in man, at all events. 



PSYCHOLOGICAL ASPECTS OF VISION. 



It is impossible to ignore entirely, in treating of the physi- 

 ology of the senses, the mind, or perceiving ego. 



By virtue of our mental constitution, we refer what we '' see " 

 to the external world, though it is plain that all that we per- 

 ceive is made up of certain sensations. 



We recognize the " visual field " as that part of the outer 

 world within which alone our vision can act at any one time ; 

 and this is, of course, smaller for one than for both eyes. 



If one takes a large sheet of paper and marks on its center 

 a spot on which one or both eyes are fixed, by moving a point 

 up or down, to the right or the left, he may ascertain the limits 

 of the visual field for a plane surface. The visual field for both 

 eyes measures about 180° in the horizontal meridian ; for one 

 eye about 145° ; and in the vertical meridian 100°. 



After-Images, etc. — Positive after-images have already been 

 referred to ; but an entirely different result, owing to exhaus- 

 tion of the retina, may follow when the eye is turned from the 

 object. If, after gazing some seconds at the sun, one turns 

 away or merely closes the eyes, he may see black suns. In 

 like manner, when one turns to a gray surface after keeping 

 the eyes fixed on a black spot on a white ground, he will see a 

 light spot. Such are termed negative after-images, and these 

 may themselves be colored, as when one turns from a red to a 



