668 THE EYE IN EVOLUTION 



a unitary perception based upon estimates of the extent of 

 the retinal image and its distance away. 



Such perceptions depend on a number of physiological and 

 psychological factors. The primary factor in a visual analysis of space 

 is the characteristic of local sign in the retinal elements — that innate 

 property, possessed by all distinguishable parts of the body, whereby 

 the excitation of one receptor is discriminated from the excitation of 

 its neighbours, so that all object -points are projected visually with 

 regard to the eye as spatial entities separate from all other points. In 



FIXATION 

 SPOT 



CENTRE 

 OF GRAVITY 



Fig. 797. — The Frame of Refebence in Space 



The two fundamental coordinates — the vertical determined gravita- 

 tionally froin the earth's centre and the horizontal determined visually from 

 the horizon. 



animals possessed of an area centralis or fovea, ^ this region is pre- 

 eminently possessed of local sign and forms the primary point of 

 reference ; images formed thereon are projected along a central base 

 line (the fixation line) in relation to which images falling on eccentric 

 retinal points are correspondingly located. Such a mechanism is 

 applicable to each eye separately, but when the visual fields overlap, 

 within the area common to both, sensory impressions are S3aithesized 

 into a unity so that objects in space are projected along a line of 

 direction emanating from a hypothetical, centrally situated cyclopean 

 eye. It is the simultaneous presentation to consciousness of two 

 slightly dissimilar images in this way that forms the basis of stereo- 

 scopic vision. In addition to this retinal mechanism orientating objects 

 in space with reference to the eyes and establishing an egocentric 



1 p. 657. 



