THE ANATOMICAL BASIS 17 



fovea or peripheral, colour vision is also affected. Foveal or central 

 vision must therefore be clearly distinguished from peripheral or eccentric 

 vision. 



Vision is profoundly affected by the condition of the retina at the 

 moment of stimulation. The condition of the retina at any given part 

 is determined by two factors, temporal and spatial. The temporal 

 factor is the nature of the stimulation to which the retina has been 

 previously submitted (temporal induction). If the eye has been exposed 

 to bright light it is said to be light-adapted. I shall speak of vision 

 under these circumstances as photopia, and the light-adapted eye as a 

 photopic eye. If the eye has been kept completely free from light for 

 a considerable period it is said to be dark-adapted. I shall speak of 

 vision under these circumstances as scotopia (as an equivalent for the 

 German Ddmmerungssehen), and the dark-adapted eye as a scotopic 

 eye. 



The spatial factor in retinal sensibility is the reciprocal action of 

 different areas upon each other. The excitability of a given area is 

 affected by the condition of sensibility and stimulation of the surround- 

 ing areas (spatial induction). 



CHAPTER III 



THE PSYCHOLOGICAL BASIS 



The physiology of the senses may be regarded as the intermediate 

 link between the outer world and the consciousness of the individual. 

 The physical stimuli come into contact with the sense organs and set 

 up sensory neural physiological changes, which may be transformed 

 into, or at all events accompanied by, mental processes leading to 

 sensation, perception, and comparison. In this long and complicated 

 path there are two points of contact, the physico-physiological and the 

 physiologico-psychological. Of the former we know a Httle, and the 

 elements of our knowledge have been discussed in the last chapter. 

 Of the latter we know nothing, but of the final perfected perceptions we 

 have some knowledge, the elements of which we must now discuss. 



The awakened perceptions, their qualities and attributes, and their 

 inter-relations belong to the domain of psychology. They depend in 

 some occult manner upon the sensations aroused by the physiological 

 processes which are set in activity by the physical stimuli. We have 

 good reason to think that there are many gradients and junctions upon 

 p. o. \ . 2 



