RECURRENT VISION 95 



increased in geometrical proportion the rapidity of stimulation increases 

 in arithmetical proportion. Griinbaum found that with high intensities 

 the law fails, just as he found the Talbot-Plateau law to fail. If light 

 alternates with less light the point of fusion is lowered by the diminution 

 of the differences of intensity. 



Filehne 1 , Schenck 2 and others found that with rotating discs the 

 number of sectors influences the result. The more numerous the sectors 

 the greater the number of rotations necessary per second for fusion. The 

 difference is less marked when the disc is viewed through a slit (Schenck) 3 . 

 Griinbaum worked out the relations and found the explanation in 

 successive contrast (vide infra). Sherrington with coloured discs found 

 the results markedly affected by simultaneous contrast (vide infra). 

 We may therefore conclude that " the point of fusion of intermittent 

 stimuli, so as to produce a continuous sensation, depends, not on the 

 physical intensities of the stimuli, but on their physiological intensities, 

 as determined by the condition and nature of the stimulated retina " 

 (Rivers) 4 . 



This fact is further borne out by the areal and regional differences. 

 Exner 5 found that the duration of the sensory process decreased in 

 arithmetical proportion as the size of the retinal image increased in 

 geometrical proportion ; and Charpentier 6 similarly found that increase 

 in the size of the area of retina stimulated raised the point of fusion. 

 Exner found that the fusion frequency was less for the fovea than for 

 a region T33 mm. outside it, and all agree that flicker persists longer 

 in the peripheral than in the central areas of the retina. Bellarminoff 7 

 found differences between the nasal and temporal areas. 



T. C. Porter's researches have increased the knowledge of flicker 

 phenomenon materially. Using black and white sectors illuminated 

 by pure spectral lights from the interference spectrum of lime-light, 

 he found that the speed at which flicker was abolished for yellow was 

 nearly double that for violet ; green and the last distinctly visible 

 red occupying the mid-position. Having regard to the increase in 

 speed with increasing intensity of the light, he concluded that as the 

 retinal stimulus increases in intensity the sensation produced retains 

 its maximum for a shorter and shorter time. With constant illumina- 

 tion, altering the relative sizes of the white and black sectors, he found 



1 Arch. f. Ophth. XXT. 2, 20, 1885. 2 Arch.f. d. ges. Physiol. cxn. 1, 1906. 



3 Ibid. LXIV. 165, 1896. 4 In Sckafer's Text Book of Physiology, n. 1072, 1900. 



5 Silz. d. k. Akad. d. Wisscnsch. Wien, LVIII. 2, 601, 1868. 



6 Arch. d'Opht. x. 340, 1890. ' Arch.f. Ophth. xxxv. 1, 25, 1889. 



