220 ILLUSIONS EXPLAINED. 



are capable of exciting in our minds. What takes place at 

 such a time in the action of the sun's rays upon the drops 

 of falling rain constitutes one of the most wonderful phe- 

 nomena in nature one the possibility of which would be 

 utterly inconceivable by us if there were not the most ir 

 refragable proof of the reality of it. 



Now the manner in which these descending drops sepa- 

 rate the sun's rays falling upon them, and send the differ- 

 ent portions radiating in various directions in such a man- 

 ner as to form upon the retina of the eye of any person 

 looking at the cloud the image of a rainbow, is somewhat 

 difficult to be explained. Sometimes a picture of a rain- 

 bow is made, with lines to represent the course of the rays 

 drawn from it to the eye of an observer on the ground, 

 who is also represented in the picture. But such a diagram 

 tends rather to confuse the ideas of the student than to aid 

 him, first, because it represents the bow on the cloud as a 

 reality, when there is no such reality there, and, secondly, 

 because the image of it in the eye could never appear to 

 be in the same position for the observer shown in the land- 

 scape and for the person looking at the picture. The rain- 

 bow is sometimes even represented as foreshortened by 

 perspective, as if it were a solid arch of many colors built 

 into the sky. Now the idea of a rainbow foreshortened, as 

 if seen obliquely, if not involving a contradiction in terms, 

 is certainly a philosophical absurdity. 



I shall therefore endeavor to give the reader some idea 

 of the general principle on which the rainbow image is 

 formed in the eye by means of the drops of falling rain, 

 without any engraving to illustrate it, though I have sev- 

 eral at hand made expressly for the purpose. I only ask 

 the reader to imagine himself to be looking out at the door 

 toward the east on a summer afternoon, just after a shower 

 has passed over, and the cloud lies in the eastern sky, while 



