D URA TION OF STIMUL US AND SENS A TION 1 069 



that a disc, such as in Fig. 386, on rotation gave a uniform grey, and that 

 the brightness of the grey was the same as that of a surface obtained by 

 mixing the double images of black and white strips, doubled by means of 

 a piece of Iceland spar. 1 The law has been found to be approximately 

 true for moderate intensities, though 

 Fick 2 found that there were slight 

 deviations. For low intensities con- 

 tinuous illumination gave a brighter 

 sensation than intermittent ; with 

 stronger illumination, intermittent 

 illumination had the greater effect, 

 while at the upper limit of intensity 

 employed by him the law held good. 

 Investigators of this question have 

 usually employed discs seen by re- 

 flected light. 0. Grunbaum, 3 using 

 direct light, has found that the law 

 does not hold above a certain intensity, 

 the brightness of a light diminished in Fig. 386. 



intensity by rotating before it alternate 



open and black sectors being greater than that of the same light dimin- 

 ished to an equal degree in physical intensity by means of Nicol's prisms. 



Flicker sensation. — When the intermittent stimulation is not 

 sufficiently rapid to cause complete fusion, a characteristic flickering 

 appearance is seen, and the point at which this flicker passes into 

 a continuous sensation has been used as a means of determining 

 the conditions which influence visual persistence. The flickering ap- 

 pearance alters in character with the rate of stimulation. With slow 

 rates of rotation of a disc divided into sectors, these can at first be 

 distinguished ; increase of the rate produces a coarse and unpleasant 

 flicker, accompanied at certain rates by a peculiar glittering appearance, 

 and this gradually passes into a fine tremulous appearance before 

 complete fusion occurs. Some observers have used the point of cessa- 

 tion of the coarse, others of the fine flicker, and it has not yet been 

 clearly established that the two points of transition follow the same 

 laws. It is difficult to make a clear distinction between coarse and fine 

 flicker, and the point of cessation of the latter is that with which the 

 most satisfactory results may be obtained. 



The point of fusion depends in the first place on intensity ; being- 

 raised by increase of intensity, i.e. more rapid stimulation is required. It 

 has been supposed that as the intensity is increased in geometrical 

 progression, the rapidity of stimulation necessary for fusion increases in 

 arithmetical progression, so that with high luminosities the point of 

 fusion is not appreciably affected by further increase. Griinbaum has 

 found that this is only true up to a certain limit, and that as intensity 

 is increased, a point is reached at which the point of fusion begins to 

 fall. This only occurs when the stimulus has reached a considerable in- 

 tensity, corresponding with that at which the same observer finds the 

 Talbot-Plateau law no longer holds good. It seems probable that both 

 changes are connected with the prolonged positive or blinding image 



1 "Handbuch d. phys. Optik," 2te Aufl., S. 483. 



2 Arch. f. Anat., Physiol, v. vnssensch. Med., 1863, S. 754. 



3 Journ. Physiol., Cambridge and London, 1898, vol. xxii. p, 433. 



