THE SENSES 939 



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 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 little 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 inter- 

 mediate 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. Instead 

 of the long and short rays falling together on the same elements 

 of the retina, as they did in the absence of the prism, they now 

 fall, if proper precautions have been taken to secure a pure 

 spectrum, in regular order from one side to the other of the 

 portion of retina on which the image is formed. The physical 

 condition, then, of our sensations of the prismatic colours is, 

 that rays of approximately the same wave-length should fall 

 unmixed with other rays upon the retinal elements. Rays of 

 a wave-length of 760 /4//,* to 650 /JL/J, give the sensation of red ; 

 from 650 //,/z, to 590 fifjL, the sensation of orange ; from 430 pp 

 to 400 fjbfju, the sensation of violet, and so on. When rays of 

 * fj./j. is a symbol representing one-millionth of a millimetre. 



