820 A MANUAL OF PHYSIOLOGY 



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 -ginrJinnrth of a second, repeated at inter- 

 val of TTijth 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 

 (Griinbaum). 



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 successive 

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 maintains that as soon as the light is withdrawn the retinal excitation 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

 but 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 

 regular intervals in the white portion of the disc. The explanation is 

 this. At the moment when the image of the advancing edge of the white 

 quadrant falls upon the retina it is excited, and we get the sensation 

 of white. Then comes a swing in the opposite direction which gives 

 rise to the first black band, and succeeding swings cause the other 

 bands. The period of the oscillatory process can be calculated from 

 the speed of the disc, and the distance of the first band from the 

 edge of the white quadrant. The well-known fact that a single flash 

 of lightning, or other intense stimulus, may appear as two flashes, 

 finds its explanation in these retinal oscillations. 



Colour Vision. Besides differences in the distance, size, 

 shape, and brightness of objects, the eye recognises differ- 

 ences in their colour ; and we have now to consider the 

 physical and physiological differences on which these depend. 



Colours may differ from each other (i) In tone or hue, e.g., red, 

 yellow, green. (2) In degree of saturation or fulness or purity, i.e.. 



