VISION 1051 



as soon as the light is withdrawn the retinal excitation begins to sink, 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. Pick'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 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 oscil- 

 latory 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 ^s two flashes, finds its explanation in these retinal oscilla- 

 tions. 



Colour Vision. Besides differences in the distance, size, shape, 

 and brightness of objects, the eye recognizes differences 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., in 

 the degree in which they are free from admixture with white light, e.g., 

 a ' pale ' or ' light ' blue is a blue mixed with much white light, a ' deep ' 

 or ' full ' blue with kttle or none. (3) In brightness or intensity, i.e., in 

 the amount of the light coming from unit area of the coloured object. 

 Thus, a ' dark ' red cloth sends comparatively little light to the eye, a 

 ' bright ' red cloth sends a great deal. 



When a beam of sunlight falls into the eye, a sensation of ' white 

 light ' results. When a prism is placed before the eye, the sensation 

 is entirely different ; we see a spectrum running up from red through 

 green to violet, with a multitude of intermediate shades, the eye 

 being able to distinguish in the solar spectrum at least one thousand 

 different hues (Aubert). What, then, has happened ? Physically, 

 nothing more has taken place than a rearrangement of the rays 

 in the beam of white light. A few of them may have been lost by 

 reflection, but upon the whole the beam is made up of exactly the 

 same constituents as before; only the rays are now arranged in the 

 precise order of their refrangibility, the more refrangible, which are 

 also those of shortest wave-length, being displaced more towards the 

 base of the prism than the longer and less refrangible rays. In- 



