1911] 071 Bioloiiy and the Cinemato(inq)h. 169 



conu'cictiuii and excitation of the retina. The muscular contraction 

 outlasts the duration of the stimulus exciting it : the curve of con- 

 traction rises slowly, reaches its height, and then declines — the dura- 

 tion of a simple twitch of a frog's muscle being about one-tenth of a 

 second, 



The retinal curve of excitation follows a somewhat similar course, 

 while the retinal excitation outlasts the luminous stimulus by about 

 one-eighth of a second. 



Flicker — Fusion of Effects. 



This " after-effect " or " after-vibration " is more or less true of all 

 our sense-organs. The direct immediate effect outlasts the duration 

 of the stimulus. This is the basis of corporal punishment administered 

 through the sensitive cutaneous nerves. 



When a given retinal area is stimulated by intermittent stimuli at 

 a certain rate, the disagreeable sensation of " Flicker " is set up, 

 which is due to a rhythmical unpleasant rise and fall of the luminous 

 sensations comparable to an " Incomplete Tetanus." If the rate of 

 stimulation is increased, there is fusion of the individual stimuh by 

 the visual apparatus — or retino-cerebral mechanism — and the result is 

 a continuous sensation of light, such as is seen when a rocket is dis- 

 charged or Catherine wheels revolve, or when a lighted stick is rapidly 

 whirled round. 



Exner has shown that if the light stimulus increases in geometrical 

 progression, the time during which it must act to produce its maximum 

 effect decreases nearly in arithmetical progression, and this time is 

 about one-quarter of a second. For fusion of the effects of luminous 

 stimuli, however, they must follow each other at a more rapid rate 

 than four per second. The results of stimulation of the retina have 

 been formulated in the form of a law, that bears the name of " Talbot's 

 Law," or " Talbot-Plateau Law." It is found tliat when once flicker 

 is established any further increase in tlie rate of rotation of the disk 

 produces no change in the character of the sensation. 



The fact, then, that our retino-cerebral apparatus can fuse, or 

 summate the effects of intermittent stimuli, falling on the retina so 

 as to give rise to the sensation of a continuous luminous impression, 

 is the basis of cinematography. Fusion of stimuli occurs with ex- 

 traordinary readiness in visual stimulation — in this respect presenting 

 a marked couti'ast to our auditory sensations. 



Stimuli of adequate frequency cause an uninterrupted visual 

 sensation to be produced. The fusion of sufficiently rapid inter- 

 mittent stimuli is easily demonstrated on the colour wheel. To get 

 rid of flicker and to obtain complete fusion, it is found that the speed 

 of rotation ilepends on the intensity of the illumination as well as 

 on the brightness value ol' the stimulus. The more intense the 

 luminous sensation, the greater must be the number of revolutions to 



