io 7 6 VISION. 



It is doubtful whether the coloured bands can be explained in 

 this way, although the distinctness of the red in Bidwell's experiments is 

 to be expected if this colour reaches its maximum with the rapidity 

 ascribed to it by Kunkel. 



Theory of fusion with intermittent stimulation. At first sight 

 the most obvious explanation of fusion of discontinuous stimuli, so 

 as to produce a continuous sensation, is derived from the analogy 

 of muscular tetanus. The rise and fall of the sensory curve takes an 

 appreciable time, and if a new stimulus comes in before the curve due 

 to the previous stimulus has had time to fall, the result would be a 

 continuous line ; the occurrence of nicker would then be due to the fact 

 that a slower rate allowed the individual curve to fall slightly before the 

 second stimulus arrived. If this were the case, however, the intensity 

 of the resulting sensation, with complete fusion, should be the same as 

 that of each individual sensation at or near to its maximum. The fact 

 known as the law of Talbot and Plateau seems therefore to negative 

 this simple view, and renders necessary some other explanation. In order 

 to reconcile the fusion of intermittent stimuli with the Talbot-Plateau 

 law, it seems necessary to adopt the assumption that a less intense 

 stimulus, occurring at a certain interval after a more intense stimulus, 

 acts immediately in preventing the excitation set up by the former from 

 reaching its maximum, and leads to a state of oscillation about a mean 

 excitation, which may produce the appearance of flicker, or if slight 

 enough may be unnoticed (remain below the threshold), and lead to the 

 appearance of continuity in the sensory process. 



As in the case of after-image, persistence has been ascribed to both 

 peripheral and central processes. Exner l found that with intermittent 

 electrical stimulation of. the eyeball, light sensations were still discon- 

 tinuous with a rapidity of sixty per second, while, with stimulation by light, 

 fusion took place with about twenty-four stimuli per second. Assuming 

 that the electrical stimuli acted on the optic nerve fibres, Exner regarded 

 the above fact as proof of the retinal origin of persistence. Filehne 2 

 objected that, under certain conditions, sixty or more light stimuli per 

 second give a discontinuous sensation; and Griinbaum 3 has shown that the 

 figure is still higher. Exner 's proof, then, loses its validity ; and there is at 

 present no definite evidence as to the seat of visual persistence. Sherring- 

 ton has shown the dependence of persistence on spatial induction or con- 

 trast, and, consequently, if the latter is central, the former must be so also. 

 The seat of induction generally, however, is also still an open question. 



THE BRIGHTNESS OF COLOUR SENSATIONS. 



Colour photometry. Numerous methods have now been devised 

 to enable different colours to be compared in respect of brightness. The 

 difficulty in doing this is enhanced by the fact that the relative bright- 

 ness of different colours varies with the absolute intensity, a fact first 

 noted by Purkinje, and usually known by his name. 



The most widely employed method of colour photometry has been the simple 

 direct comparison of coloured with colourless light. This method was employed 

 by Fraunhofer in 1817, and the extensive researches of Abney 4 and Kb'nig 5 on 



1 Arch.f. d. ges. PhysioL, Bonn, 1875, Bd. xi. S. 414. 



2 Arch.f. Ophth., 1885, Bd. xxxi. Abth. 2, S. 20. 



3 Journ. PhysioL, Cambridge and London, 1897, vol. xxi. p. 398. 



4 Phil. Trans., London, 1886, vol. clxxvii. p. 423 ; and 1888, vol. clxxix. p. 547. 

 6 "Beitr. z. Psych, u. Phys. d. Sinn.," 1891, S. 311. 



