BECOMPOSITION OF LIGHT. 



bottom with those forming the lower end of the spectrum, red, 

 orange, and yellow. This effect is explained by the fact, that 

 until the screen is brought to the focus F, the extreme rays at 

 the other end of the spectrum arc not combined with the other 

 colours. 



If the screen be removed beyond F, the same succession of 

 appearances will be produced upon it as were exhibited in its 

 approach to F, but the colours will be shown in a reversed 

 position. 



As the screen leaves F, the white spot upon it is fringed as 

 before, but the upper fringe is composed of red, orange, and 

 yellow, while the lower is composed of violet, blue, and green ; 

 and when the screen is removed so far from the focus F as to pre- 

 vent the superposition of the colours, the spectrum will be 

 produced upon it, with the red at the top, and the violet at the 

 bottom, the position being inverted with respect to that which the 

 screen exhibited at the other side of the focus. These circum- 

 stances are all explained by the fact that the rays converging to F 

 intersect each other there. 



20. Similar, effects may be produced by receiving the spectrum 

 upon a double convex lens, as represented in fig. 9. The rays 



Fig. 9. 



are made as before to converge to a focus F, where a white spot 

 would be produced upon the screen. Before the screen arrives at F, 

 and after it passes it, the same effects will be produced as with 

 the concave reflector. 



21. The proposition, that the combination of colours exhibited 

 in the prismatic spectrum produces whiteness, may be further 

 verified by the following experiment : 



Let a circular card be framed with a blackened circle, and its 

 centre surrounded by a white circular band, and a black external 

 border, as represented in fig. 11. 



Let the white circular band be divided into seven spaces pro- 

 portional in magnitude to the spaces occupied by the seven colours 

 in the prismatic spectrum, these spaces being B, o, Y, G, B, I, and v. 

 Let these spaces be respectively coloured with artificial colours 

 resembling as near as practicable in their tints the colours of the 



71 



