22 PHYSICAL SCIENCE BK. i 



cloudy as to hide the sun. It must, therefore, 

 unquestionably arise from these, failing either of 

 which it cannot come into being. 



IV 



A FURTHER consideration must be mentioned, which 

 is just as manifest as the preceding, to prove 

 that the reflection is given back after the fashion 

 of a mirror ; it is never given back save from 

 straight opposite to the sun, that is, 1 unless on 

 one side stands the object to be reflected, and on the 

 other the mirror that reveals it. Proofs are adduced 

 by the mathematicians that are not merely convinc- 

 ing but that compel belief of this. Nor can doubt 

 be left in any mind that the rainbow is an image of 

 the sun, imperfectly reflected owing to the defective 

 shape of the mirror. But meantime let us 



recall other proofs that may, so to speak, be picked 

 up in the street without any reference to mathe- 

 matics. Among the proofs of this origin of the bow 

 I place the extreme rapidity of its emergence. In 

 a single moment the huge form with its thousand 

 lines is inwoven in the texture of the heavens, and 

 just as rapidly does it fade. Now, nothing is 

 returned so quickly as an image from a mirror. 

 The mirror does not create anything, it merely 

 reveals it. Artemidorus of Parium tells us further 

 even the kind of cloud required to reflect such an 

 image of the sun. If you make a concave mirror, 

 he says, that is, one resembling half of a ball cut 

 through the middle, and take your stand outside 

 the centre, then those who stand beside you will 



1 In a writer less prone to repetition the words to the end of the sentence 

 would seem the insertion of a copyist. 



