AND ITS CONGENERS. 203 



latter needed explanation just as much as the colours 

 of the bow itself. The solution, indeed, was not possible 

 until the composite nature of white light had been 

 demonstrated by Newton. Applying the law of Snell 

 to the different colours of the spectrum, Newton proved 

 that the primary bow must consist of a series of concen- 

 tric circular bands, the largest of which is red and the 

 smallest violet ; while in the secondary bow these colours 

 must be reversed. The main secret of the rainbow, if 

 I may use such language, was thus revealed. 



I have said that each colour of the rainbow is 

 carried to the eye by a sheaf of approximately parallel 

 rays. But what determines this parallelism ? Here 

 our real difficulties begin, but they are to be surmounted 

 by attention. Let us endeavour to follow the course 

 of the solar rays before and after they impinge upon a 

 spherical drop of water. Take, first of all, the ray that 

 passes through the centre of the drop. This particular 

 ray strikes the back of the drop as a perpendicular, its 

 reflected portion returning along its own course. Take 

 another ray close to this central one and parallel to it 

 — for the sun's rays when they reach the earth are 

 parallel. When this second ray enters the drop it is 

 refracted ; on reaching the back of the drop it is there 

 reflected, being a second time refracted on its emergence 

 from the drop. Here the incident and the emergent 

 rays enclose a small angle with each other. Take, again, 

 a third ray a little further from the central one than 

 the last. The drop will act upon it as it acted upon 

 its neighbour, the incident and emergent rays enclosing 

 in this instance a larger angle than before. As we 

 retreat farther from the central ray the enlargement of 

 this angle continues up to a certain point, where it 

 reaches a maximum, after which farther retreat from 

 the central ray diminishes the angle. Now, a maximum 

 U 



