OF OPTICS. 277 



fracted into the line b c as 



they enter the drop, and are 



reflected from the back of it 



at c, in the line c d, and then, 



by passing out of the drop 



into air, they are again refracted at d; and from 



thence they pass on to the eye et e : so that to form the 



interior bow, the rays suffer two refractions, as at b and 



d ; and one reflection, as at c. 



The exterior bow is formed by rays which suffer two 

 reflections, and two refractions ; which is the occa- 

 sion of its being less vivid than the interior, and 

 also of its colours being inverted with respect to 

 those of the interior. For, when a ray a b falls 

 upon the lower part of the drop 

 b c d e, it is refracted into the 

 direction b c by entering the 

 drop ; and passing on to the 

 back of the drop at c, it is 



thence reflected in the line c J, in which direction 

 it is impossible for it to enter the eye at f: but by 

 being again reflected from the point d of the drop, it 

 goes on in the drop to e, where it passes out of the 

 drop into the air, and is there refracted downward to 

 the eye, in the direction ef 



