288 ADAPTATIONS TO SPACE AND MOTION 



also in an increase of the hypermetropia from a half-diopter or so to 

 five, seven, even ten diopters. This situation has been branded as a dis- 

 harmony, supposedly inevitable in small eyes simply because they are 

 small. This notion ignores the optical perfection of even smaller fish 

 eyes. The apparent disharmony simply reflects the indifference of mice 

 and mouse-sized mammals in general to any refinements of vision relat- 

 ing to resolving power. The cerebral images of mice and the like are so 

 crude at best, that the eye is useful more for recording the intensity and 

 direction of light, and the motion of large objects in the visual field, 

 than for discrimination of pattern. In such animals, the 'nose knows' 

 far more about the environment than does the eye. 



(B) Visual Angles and Fields 



In all vertebrates, vision predominates in any accurate localization of 

 objects in space. Aside from vision, only audition and olfaction are 

 telassthetic senses — that is, capable of giving information about objects 

 and events at a distance. The distance and direction of an object which 

 is beyond arm's reach can be only crudely judged by these other tel- 

 aesthetic modalities, and can be accurately evaluated only through vision 

 if at all. Audition is notoriously untrustworthy as a means of localization. 

 The finding of an object by olfaction is a trial-and-error process, and is 

 not localization at all in the sense of a pre-knowledge of location. 



The visual registration of space entails the embracement, by the retinae, 

 of light rays coming from many directions. The animal may be thought 

 of as having its head at the center of a sphere of space. The proportion 

 of that sphere within which the animal can see is influenced by several 

 factors : 



A. The visual angle, in various meridians, of each eye; 



B. The position of the eyes in the head and the ratio of binoc- 

 ular field to total visual field; 



C. The orientation of the visual axes, where these do not coin- 

 cide with the anatomical optic axes; 



D. The capacity for reflex and voluntary eye movements and 

 the location of the area centralis or fovea, if one is present; 



E. The capacity for head movements in compensation for any 

 severe reduction of visual angle or eye mobility. 



