SECTION IX 



THE SUBJECTIVE PHENOMENA OF VISION 



BY ' subjective ' we mean that the sensations under consideration cannot 

 be directly traced to the stimulus which initiated them. Thus 

 at a certain rate, intermittent stimuli presented to the eye form 

 a continuous sensation, so that nicker appears to have ceased. But 

 the carrying on of the sensation from one stimulus to the next is performed 

 by some part of the visual mechanism, and has nothing to do with any physi- 

 cal peculiarity of the light. It is therefore an example of a subjective 

 phenomenon. 



ORDINARY CONE RESPONSE. 



AFTER. IMAGE. 

 CONE RESPONSE. 



FIG. 291. Curve representing diagrammatically the sensations aroused when the 

 eye has been stimulated by a flash of light. Intensity of sensation-vertical. 

 Time-horizontal. (HARTRIDGE.) 



THE SENSATION CURVE. When a light 

 stimulus enters the eye a certain period of 

 time elapses before a sensation is perceived. 

 Tlhis latent period may be compared with that 

 which occurs between the stimulus and con- 

 traction of a muscle. After its commencement 

 the sensation rapidly rises to a maximum (see 

 Fig. 291) and then shows several rapid fluctua- 

 tions as it reaches its mean value. These 

 fluctuations are called Charpentier's bands, and 

 are well seen after stimulating the eye by 

 means of the flash from an electric spark. 

 They have been compared to the oscillations 

 which occur when an electric current is passed 

 down a telegraph cable, and which are caused 

 by the inductance and capacity of the circuit. 

 566 



FIG. 2!tL'. Charpentier's bands 

 as seen \\hen a disc with a 

 iiarmw radial slit is rotated 

 in front of an illuminated 

 screen about once a second. 



