1913.] Sensations and the Theory of Forced Vibrations. 499 



The effect of a sudden flash must be to set free a definite quantity of 

 exciting molecules which will continue to produce sensation until their 

 power is exhausted, or they are dispersed. This may occupy several minutes 

 ■even after a momentary flash, and much longer for a more prolonged exposure 

 to light. The phenomenon may be detected as a very brief after-effect even 

 with light of low intensity. 



Using the terms already employed : — 



dx/dt is conditioned by the intensity of the illumination and the store n 



of exciting molecules. 

 de/dt represents the positive after effect so soon as dx/dt becomes zero, i.e. 



when exciting molecules are no longer produced. 



This agrees with Fechner's theory as described by Helmholtz. And at this 

 point it becomes necessary to take into account also the pbysiological elements 

 •of the visual sensation. 



II. Physiological Elements of the Visual Sensation. 



Thus far, the conditions discussed relate to one process only of the opto- 

 chemical action, namely, the setting free of the active decomposition products 

 which excite the retina from the bland visual substances which have no 

 action upon it. 



The other process, necessary to complete the visual effect, is physiological. 

 It is complex, including the excitation of the sensitive elements of the 

 retina by the active decomposition products, the transmission to the central 

 organ of the response, and its translation into conscious sensation. Moreover, 

 there is strong evidence of the existence of a protective mechanism whereby 

 the intensity of the stimulus is regulated. 



This physiological process is curiously distinct from the first-described 

 ■op to- chemical process. The rate at which the exciting substance can be 

 produced far exceeds that at which it can be used up. This can easily be 

 shown by means of a photographic exposing shutter fixed in the window of 

 the dark room. The shutter is set to give an exposure of known length, and 

 the observer looks through it at the sun's disc reflected in the mirror of the 

 heliostat. The positive after-image so produced lasts many times longer than 

 the flash. I have made some attempts to establish a relation between length 

 of flash and duration of after-image, but have had to relinquish the work for 

 the present before getting complete data. 



The store of exciting molecules set free by the action of the light is used 

 ^up at a rate depending apparently on the concentration, so that it is rapid at 

 first and dwindles down to nothing. It follows from this that during: 



