SCIENCE OF COMMON THINGS. 259 



Colors in a soap-bubble. Origin of morning and evening twilight. 



the rays which enter at the top of the rain-drops, 

 refracted from the bottom : 



But in the other bow we see the rays which enter at 

 the bottom of the rain-drops (after two reflections) 

 refracted from the top. 



The position and formation of the primary and secondary rainbows are 

 represented in Fig. 73. Thus in the formation of the primary bow, the 

 ray of light S strikes the drop n at a, is refracted to n &, thence to g, and 

 leaving the drop at this point, proceeds to the eyes of the spectator at 0. 

 In the formation of the secondary bow, the ray S' strikes the drop p at the 

 bottom at the point i, is refracted to rf, thence to/, and again to e, pro- 

 ceeding from the top of the drop, also to the eye of the spectator at 0. 



The reason why the primary bow exhibits the stronger colors is, be- 

 cause the colors are seen after one reflection and two refractions ; but the 

 colors of the secondary (or upper) rainbow undergo two reflections and 

 two refractions. 



Fig. 73. 



1688 Why does a soap bubble exhibit such a variety of colors f 

 Because the thickness of the film through which the 

 rays pass is constantly varying. 



1680 Why is a soap bubble so constantly changing its thickness t 



Because the water runs down from the top to the 

 bottom of the bubble, till the crown becomes so thin as 

 to burst. 



3 60O Wliat is the cause of morning and evening twilight f 



When the sun is below the horizon, the rays which 

 strike upon the atmosphere or clouds are bent down 



