938 A MANUAL OF PHYSIOLOGY 



there is no stimulation, the sensation, when complete fusion has been 

 reached, is the same as would be produced by a constant light of 

 half the strength employed. And, in general, if m be the proportion 

 of the time during which the eye is stimulated by a light of intensity 

 7, and n the proportion of the time during which it is not stimulated, 

 the resultant impression is the same as that which would be produced 



by an uninterrupted light of intensity ( V. This is Talbot's 



law, which may be expressed without the aid of symbols thus : When 

 a light of given intensity is allowed to act on the eye at intervals so short 

 that the impressions are completely fused, the 

 resultant sensation is independent of the abso- 

 lute length of each flash, and is proportional 

 only to the fraction of the whole time which is 

 occupied by flashes and to the intensity of the 

 light. Talbot's law may be readily demon- 

 strated by means of a rotating disc with alter- 

 nate white and black sectors (Fig. 412), so 

 arranged that the same proportion of the cir- 

 cumference of each of the three concentric 

 zones is black. 



FlG 4I2 Disc FOR DE- When the rotation is sufficiently rapid to 



MONSTR ATI NG TALBOT'S give complete fusion (say 20 to 30 times a 



LAW. second), the whole disc appears equally bright. 



However much the rate of rotation is now 



increased, no further change occurs. It has been shown that even 

 for stimuli as short as the HOOOOOO^ f a second, repeated at intervals 

 of yfo'th second, Talbot's law holds good. So that not only does 

 a flash so inconceivably brief affect the retina, but it sets up changes 

 which last for a measurable time. For intense stimuli Talbot's law 

 ceases to be true : the field appears brighter than it should be 

 (Grunbaum) . 



Two chief theories have been proposed to account for the fusion of 

 intermittent retinal stimuli : (i) The persistence theory, according to 

 which the excitatory process in the retina remains for a short time 

 at the maximum reached when the light ceases to act. Steady 

 fusion is supposed to be obtained when the interval between succes- 

 sive stimuli does not exceed this time. (2) The theory of Fick, who 

 maintains that as soon as the light is withdrawn the retinal excitation 

 begins to sink, at first rapidly, then more gradually. As the rate of 

 stimulation is increased the time allowed for the decline of the 

 excitation is, of course, correspondingly shortened, and ultimately 

 the oscillations become so small that a continuous smooth sensation 

 results. Fick's theory appears to explain the phenomena best. 



The experiments of Charpentier have shown that the retina when 

 stimulated has a natural tendency to enter into oscillations at the 

 rate of about 36 in the second, so that the effect of a flash of light 

 when it falls on a retinal area is not a single excitation which rises 

 smoothly to its maximum and then declines smoothly to zero, but a 

 series of swings which die away like the vibrations of an elastic body. 

 This may be demonstrated by slowly rotating a well-illuminated disc, 

 one quadrant of which is white and the rest black, while the eye is 

 kept fixed on the centre. A black band, or rather sector, running 

 out from centre to circumference, will be seen in the white quadrant 

 a little behind the border of it which first passes the eye. This 

 band may be succeeded by one or more fainter black bands placed at 



