THE RAINBOW. 413 



(6.) Paint three narrow strips of cardboard, one vermilion red, 

 one emerald green, and the other aniline violet. Be sure that the 

 coats are thick enough thoroughly to hide the cardboard. When 

 dry, hold the red strip in the red of the solar spectrum ; it appears 

 red. Move it slowly through the orange and yellow ; it grows 

 gradually darker. In the green and colors beyond, it appears black. 

 Repeat the experiment with the other two strips, and carefully 

 notice that the effects are in accordance with the principle above 

 stated. 



641. The Rainbow. The rainbow is due to re- 

 fraction, reflection and dispersion of sunlight by water- 

 drops. The necessary conditions are : 



(1.) A shower during sunshine. 



(2.) That the observer shall stand with his back to the 

 sun, between the falling drops and the sun. 



(a.) The centre of the circle of which the rainbow forms a part 

 is in the prolongation of a line drawn from the sun through the 

 eye of the observer. This line is called the axis of the bow. 



FIG. 318. 



642. Dispersion by a Raindrop. Suppose the 

 circle whose centre is at (Fig. 318) to represent the section 

 of a raindrop. A ray of sunlight, as Sm, falling upon the 

 raindrop would be refracted at m, reflected at n, and again 



