io68 VISION. 



been denied. According to Kunkel, there is a photo-chromatic interval as 

 regards duration in the case of green and blue, but not in the case of red. 



Persistence of vision. When the retina is affected by a moment- 

 ary light-stimulus of ordinary intensity, the sensory process, owing 

 to inertia, takes a certain time in reaching its maximum, and then 

 fades away, rapidly at first, more gradually later. These three 

 periods may be known as the periods of rise or waxing (Anklingeri), 

 maximum, and fall or waning (Abklingen) of the sensory curve. The 

 period of rise has been most fully investigated by Exner, 1 who used an 

 instrument devised by Helmholtz. Two surfaces were exposed in rapid 

 succession, the first, a white square on a black surface ; the second, a 

 uniform white surface. It was found that if the first excitation had 

 reached its maximum, while the second was still in the period of rise, 

 the appearance would be that of a light square on a dark ground ; if, 

 on the other hand, both had passed the maximum, the first excitation 

 would have fallen to a lower level than the second, and the appearance 

 would be that of a dark square on a light ground. Between these two 

 appearances there would be a moment at which the surface would 

 appear uniform, and this appearance would correspond to a condition 

 when one excitation had not quite reached the maximum, while the other 

 had just passed it. By this method Exner found that the time taken 

 in reaching the maximum varied with the intensity. The more intense 

 the stimulus, the more rapid the rise of the sensory curve, and the 

 relation between the two appeared to be such that if the intensity 

 increased in geometrical progression, the time in which the curve 

 reached its maximum decreased in arithmetical progression. The 

 period of waxing in the case of different colours was determined by 

 Kunkel 9i using Exner's method. He determined the times for red, green, 

 and blue; when these colours were made of approximately equal 

 brightness, he found that the period of rise was shortest for red and 

 longest for green, the times taken in reaching the maximum for red, 

 blue, or green being "0573 sec., '0916 sec., and "133 sec. respectively. 



The periods of maximum and of fall are not capable of such exact 

 investigation. By varying the intensity of his two excitations, Exner 

 endeavoured to determine the course of the curve and the total duration 

 of the sensory process, and found the maximum to be of very short 

 duration followed by a gradual fall, the total duration varying with 

 the intensity, the size of the retinal image, and the part of the retina, 

 stimulated. It is probable that the period of waning is different for 

 different colours, but there are no exact data on this point. 



Talbot-Plateau law. One of the chief methods which has been 

 employed to investigate the nature of the sensory curve is that 

 of intermittent retinal stimulation. When momentary stimuli follow 

 one another with sufficient rapidity, the effect is a continuous sensa- 

 tion of uniform brightness. The brightness of this sensation is that 

 which would have arisen if the amount of light intermittently reaching 

 the retina had been uniformly distributed over the whole period of 

 stimulation. This proposition, which is known as the law of Talbot 3 

 and Plateau, 4 has been experimentally investigated. Helmholtz found 



1 Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1868, Bd. Iviii. Abth. 2, S. 601. 



2 Arch.f. d. ges. PkysioL, Bonn, 1874, Bd. ix. S. 197. 



3 London, Edinburgh, and Dublin Phil. Mag., London, 1834, Ser. 3, vol. v. p. 327. 



4 Ann. d. Phys. u. Chem., Leipzig, 1835, Bd. xxxv. S. 457. 



