SEEING COLOURS. 



5. Looking at One Colour, and seeing Another. 



We have all seen in the monthly magazines 

 the soap advertisements which depend upon 

 staring at some strongly coloured letters ; but 

 many have not examined them with the atten- 

 tion which they deserve as paradoxical phe- 

 nomena of genuine scientific interest. 



Take from the shelves a book which is 

 bound in some strong, bright colour, such as a 

 vivid red. If there are letters printed or 

 stamped on it, mark some small point where two 

 lines cross ; if it is plain, make a very small 

 mark upon its surface. Keeping the head 

 steady and the book still, stare hard at this 

 mark for a minute or half a minute. During 

 this time do not let the gaze wander from the 

 small mark to any other part of the surface. 

 This fixity of gaze on one spot is contrary to 

 natural tendencies, and will require great deter- 

 mination. After about half a minute close the 

 eyes and cover them with the hand. An image 

 of the object looked at will be seen, but not 

 in its natural colour. 



The colour seen will be the one comple- 

 mentary to the natural one that is, the one 

 which, if compounded with the natural colour, 

 would give white. So, if the book be a bright 

 red, the after-picture will be green, and vice 

 versa. Yellow gives rise to blue, and blue to 

 yellow. 



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