COMMON THINGS COLOUR. 



Fig. 15. 



its image will be in the position f f r r ; if it be violet, its 



image will be in the position g g s s. 



Now, if we suppose the object A. A M M to be 

 white, that is to say, to have a colour which 

 combines all the prismatic colours together, then 

 all these several images will be seen at once 

 through the prism in the respective positions 

 already described. They will therefore be more 

 or less superposed one upon the other, and the 

 image will exhibit in its different parts those tints 

 which correspond to the mixture of the colours 

 thus superposed. 



Hence it appears that the space between a a 

 and b b from which all colour except the red is 

 excluded, will appear red ; in the space between 

 b b and c c, in which the orange image is super- 

 posed upon the red image, a colour will be exhi- 

 bited corresponding to the mixture of these two 

 colours ; in the space between c c and d d, the 

 three images red, orange, and yellow are super- 

 posed, and a colour corresponding to the combi- 

 nation of these will be produced. In fine, the 

 colours which are superposed between every suc- 

 cessive division of the upper and lower edges of 

 the combined images are as follows, where the 

 prismatic colours are designated by the capital 



.letters, and their mixture or superposition by the sign + : 



Between a a and 6 6 



66 



,, c c d d 



j, dd e e 



,, e e 



// 



99 



ff 

 99 



R + 



R + + Y 



R + + Y 



R + O + Y 



R + + Y 



R + + Y 



|| 



G 4- B 



O + B 4- I 



G + B + I 



Thus it appears that the space between ff g the bottom of the 

 violet image and the top m m of the red image is coloured with 

 a white light, because in this space all the seven images are 

 superposed. 



In the space between g g, the bottom of the violet image, and 



.//, the bottom of the dark blue image, there is a space which is 



illuminated by all the prismatic colours except the violet, and this 



space consequently approaches so near a white as to be scarcely 



distinguishable from if. The space between//, the bottom of the 



dark blue image, and e e, the bottom of the light blue image, is 



illuminated by all the colours except the dark blue and indigo, 



7G 



