THE EYE AS A SENSORY APPARATUS 271 



ceived. It is for this reason that we do not see the retinal vessels 

 under ordinary circumstances. When light, as usual, enters the 

 eye from the front through the pupil the shadows always fall on 

 the same parts of the retina, and these parts are thus kept suffi- 

 ciently more excitable than the rest to make up for the less light 

 reaching them through the vessels. 



Further evidence that the rod and cone layer is the true .recep- 

 tor of the eye is furnished by the fact that the seat of most acute 

 vision is the fovea centralis, where only this layer and the cone- 

 fibers diverging from it are present. When we want to see any- 

 thing distinctly we always turn our eyes so that its image shall 

 fall on the fovese. 



The Intensity of Visual Sensations. Light considered as a 

 form of energy may vary in quantity; physiologically, also, we 

 distinguish quantitative differences in light as degrees of bright- 

 ness, but the connection between the intensity of the sensation 

 excited and the quantity of energy represented by the stimulat- 

 ing light is not a direct one. In the first place, some rays excite 

 our visual apparatus more powerfully than others : a given amount 

 of energy in the form of yellow light, for example, causes more 

 powerful visual sensations than the same quantity of energy in 

 the form of violet light. 



Furthermore, the sense of vision, like all the other senses, obeys 

 the psychophysical law (Chap. XIII). That is, differences of 

 sensation are proportional not to absolute but to relative changes 

 in the amount of stimulating energy. If a room is lighted by one 

 candle and another is brought in we perceive an increase of il- 

 lumination, but if it is lighted by an arc light the bringing in of a 

 single lighted candle makes no perceptible difference in the il- 

 lumination. Another illustration of the application of the psycho- 

 physical law to the visual sense is found in the fact that the stars 

 which are ordinarily invisible in the daytime can be seen from 

 the bottom of deep wells or from deep and narrow canons. The 

 explanation is that in open day the general illumination of the 

 sky is so intense that the additional light of the stars is unper- 

 ceived. To one in the bottom of a well, however, the general 

 illumination is cut down so much as to bring the additional light 

 from the stars within the limits of perception. The smallest dif- 

 ference in luminous intensity which we can perceive is about 



