OPTICS. 



51 



there were no ground to intercept the 

 rain and the view of the observer, the 

 rainbow would form a complete circle, 

 the centre of which is diametrically 

 opposite to the sun. It will be found, 

 either by calculation or projection, that 

 the inclination of the red ray i j E to 

 the violet ray S R 'is 42<> 2', while that 

 of the violet ray v' E to S R is 40<> 17'; 

 so that the breadth of the rainbow is 

 4-2 2' - 41 17' = 145', or about 3 

 times the sun's diameter. The bow thus 

 formed is called the primary rainbow, 



and it is produced by one reflexion and 



s of the 

 drops of rain. 



two refractions of the sun's rays by the 



If the rays / E, v' E were to be a 

 second time reflected at the points r 1 

 and v', they would suffer their second 

 refraction a little below R, and would 

 entirely escape from the observer at E. 

 But though this is the case with rays 

 S R that enter at the side of the drop 



. 50. 



