32 PHYSICAL SCIENCE BK. i 



2 in this contention. My reason for saying so ? The 

 sun, even though he is on the upper side, yet 

 strikes, and therefore colours, the whole cloud. 

 How could it be otherwise ? His rays are wont 

 to be transmitted through the clouds and to 

 penetrate any density in them. Further, the proof 

 they advance is flatly in opposition to their main 

 proposition. For if the sun is higher than the 

 clouds, and his beams, therefore, shed only on their 

 upper side, the bow would never come down as far 



3 as the earth. Yet it does descend to the very 

 ground. Besides, the bow is never seen except 

 opposite to, not below, the sun. The fact is, the 

 sun s highness or lowness does not affect the 

 matter : the side of the cloud that faces him is 

 struck by him throughout its whole extent. 



Furthermore, sometimes even the setting sun 

 produces a rainbow ; surely at that time, being near 1 

 the earth, he strikes the clouds on their lower side. 

 And yet then, too, the bow is only a semicircle, though 

 the clouds receive the sunlight on their lower and 



4 darker portions. The Stoics, who hold that the 

 light is reflected in the cloud as in a mirror, make 

 the cloud hollow like the section of a ball. Such a 

 mirror, being but part of a circle, cannot, they 

 think, reproduce a whole circle. I give my adherence 

 to the proposition, but I cannot agree to the 

 argument in its support. For, if the whole figure 

 of a circle placed opposite a concave mirror is 

 reproduced in it, then there can surely be nothing to 

 prevent the whole of a ball being seen in a semi- 



5 circular mirror. Besides, we have already shown that 

 complete rings resembling a rainbow surround the 



1 The common reading makes this adjective refer to clouds the clouds 

 which are near the earth. 



