EETINAL EXCITATION 357 



If the velocity of rotation is further accelerated, the clear 

 sensation of alternate white and black ceases, and the brightness 

 of the disc assumes a mean value of intensity oscillating between 

 white and black (grey). This shows that the sensations of white 

 and black no longer have the necessary time to reach their maxi- 

 mal intensity, owing to the speed at which the stimuli appear and 

 disappear. 



If the velocity of the rotating disc is still further accelerated 

 the apparent variations in its luminosity cease, and it appears 



uniformly grey. In this case, according_to lalbot^s law, the 



luminous sensation should be the same as if the total action of 

 the light were uniformly distributed in time. In practice, how- 

 ever, the disc of white and black sectors is not of equal light 

 intensity at all velocities of rotation, but (according to Briicke) is 

 a brighter grey when its revolutions amount to 17-18 per second. 

 This striking deviation from Tal- 

 bot's law depends on the varying 

 ratio between the duration of the 

 successive stimulations and the 

 duration of the rising and falling 

 phases of the excitations. Schenck 

 further observed that if the white 

 and black are not regularly dis- 

 tributed over the sectors of the 

 disc a higher velocity of rotation 

 is required to obtain fusion of the 

 two sensations. 



When the sensations of white 

 and black are not fully fixed, that 



, y .. . FIG. 173. White and black disc of Helmholtx. 



is, when the speed of rotation is 



moderate, " nicker " sensations can also be observed, in which all 

 the colours of the spectrum are visible. White light can therefore 

 be subjectively broken up into its components by means of a 

 rotating disc with white and black sectors, without recourse to 

 prisms or similar apparatus. This is an interesting point, as it 

 shows the complicated nature of the conditions which determine 

 the reaction of the retina. 



To explain it we must start from the fact that for the simple 

 colours of the spectrum, as well as for w r hite light, a certain time 

 is required before the retinal excitation can attain its maximum. 

 The subjective decomposition of white light by the rotation of 

 white and black discs can easily be explained on the assumption 

 that the waxing and waning phases of sensation excited by rays 

 of different wave-length are of different duration. It is only by 

 assuming this that we can interpret the nicker sensation of 

 colours in rapid succession when the velocity of rotation of the 

 disc is moderate. 



2 A i 



