THE SENSES 821 



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 multi- 

 tude 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. 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 sensa- 

 tions of the prismatic colours is, that rays of approxi- 

 mately the same wave-length should fall unmixed with other 

 rays upon the retinal elements. Rays of a wave-length of 

 76onnr<r to 650 T75 ^ nr give the sensation of red ; from 

 to 590,^, the sensation of orange; from 43Onnnr to 

 the sensation of violet, and so on. When rays of all these 

 wave-lengths fall together, in the proportions in which they 

 are present in sunlight, upon the same part of the retina, 

 the resultant physiological effect is very different; we are 

 no longer able to distinguish red, blue, green, etc.; we 

 receive the single sensation of white light. The sensation 

 is a simple one ; in consciousness we have no hint that it 

 has a multiple physical cause. 



