COMPLEMENTARY COLOURS. 



Pig. 12. 



conveniently placed, or upon the ceiling of the room. The spot 

 of light thus produced will be white. 



22. Although the phenomena attending the prismatic spectrum 

 prove that rays of light which differ in refrangibility also differ 

 in colour, the converse of this proposition must not be inferred ; 

 for it is easy to show that two lights which are of precisely the 

 same colour, may suffer very different effects when transmitted 

 through a prism. 



Let us suppose two holes made in the screen on which the 

 spectrum is thrown in the middle of the space occupied by the 

 blue and yellow colours, so that rays of these colours may be 

 transmitted through the holes. Let these rays be received upon a 

 double convex lens, and brought to a focus at G', (fig. 12) upon 

 a sheet of white paper, so as to illuminate 

 the spot G'. The colour that it produces then 

 will be a green. Let another spectrum be now 

 thrown by a prism upon the screen, and let a 

 hole be made in the screen at that part of the 

 green space where the tint is precisely similar 

 to the colour produced at G' on the white paper, 

 and let the light which passes through this 

 hole fall upon the spot G beyond G'. 



The spaces G and G' will then be illuminated 

 by lights of precisely the same colour ; but it 

 will be easy to show that these lights are not 

 similarly refrangible. 



Let them be viewed through a prism having 

 its refracting angle presented upwards. The 

 image of the illuminated space G will be seen in 

 a more elevated position at g ; but two images 

 will be produced of the space G', one yellow and the other blue 

 at y and b, the yellow image y being a little below g, and the 

 blue image b a little above it. Thus it is evident that the 

 green light on the space G' is a compound of yellow and blue, 

 and is separable into its constituents by refraction, while the 

 similar green light on the space G is incapable of decomposition 

 by refraction. 



23. An endless variety of tints may be produced by combining 

 in various ways the colours composing the prismatic spectrum ; 

 indeed, there is no colour whatever which may not be produced 

 by some combination of these tints. Thus, all the shades of red 

 may be produced by combining some proportion of the yellow and 

 orange with the prismatic red ; all the shades of orange may be 

 produced by combining more or less of the red and yellow with 

 each other and with the orange ; all the shades of yellow may 



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