THE SENSES 121 



2. While the various sensations which we call colour are 

 generally produced by vibrations of different lengths falling 

 on the retina, colour sensations are also produced in 

 various other ways. 



(a) By mechanical stimulation of the retina. By pressing 

 on the eyeball as far back as possible a yellow ring, or part 

 of a ring, may often be seen. (Experiment.) 



(b) Simple alternation of white and black upon the 

 retina may produce colour sensation, as when a disc of 

 paper marked with lines is rotated rapidly before the eye. 

 (Experiment.) (Fig. 59.) 



3. By mixing different parts of the spectrum, some inter- 

 mediate part or white may be produced. This may be done 

 by colouring the surface of a top with different colours, or by 

 means of a sheet of glass allowing the image of one wafer to 

 fall on another of a different colour. (Experiment.) 



This means that by different modes of stimulation of the 

 retina the same sensation may be produced. The sensation 

 of orange may be produced either 

 when vibrations at about 580 

 billions per second fall on the 

 eye, or when two sets of vibrations, 

 one about 640 and one about 

 560 billions, reach it. By no pos- 

 sible physical combination of the 

 two is it possible to produce the 

 intermediate rate of vibration. 



The sensation of colour, there- 

 fore, depends upon the nature of 



., , .. FIG. 59. Disc which, when ro- 



the change set up in the retina, tated in a bright light, gives 

 and not upon the condition pro- impression of colours. 

 ducing that change. 



4. After looking for some time at any one colour, on 

 removing the colour another appears in its place the 

 complemental colour. If the first colour is 



Red, the second will be green blue ; 

 Orange, ,, blue ; 



Green, pink; 



Yellow, indigo blue ; 



and vice versa. (Experiment.) 



