26 PHYSICAL SCIENCE BK. i 



from him, even though they do not possess the 

 9 power of mirrors ? A little ago, my oppo- 



nent retorts, you advanced the argument that the 

 rainbow is always produced opposite the sun, be- 

 cause an image could not be reflected from a mirror 

 unless the object were in front of it. We agree 

 in this point, he adds. Yes, for just as the object 

 whose image is to be transferred to the mirror 

 must be set opposite the mirror, in like manner, 

 in order that the clouds may be tinged by the 

 sun's rays, the sun must occupy a suitable posi- 

 tion. He does not produce the same effect if 

 his light streams in on all sides ; there must 

 be a proper incidence of the rays to produce 

 the effect. Such are the reasons alleged by 



those who will have it that the rainbow is a coloured 

 cloud. 



Posidonius and those who are of opinion that 

 the phenomenon is produced by reflection as from 



10 a mirror, answer their arguments thus : If there 

 were any real colour in a bow, it would persist, 

 and be seen more distinctly, the nearer it is. As 

 it is, the image of the bow is clear only in the 

 distance ; it is lost as it begins to approach. 

 I do not agree with this argument in refutation, 

 though I approve the main sentiment which 

 it supports. And I will tell you why. The 

 cloud is coloured, but in such a way that the 

 colour cannot be seen from every point. And no 

 more can the cloud itself : for no one who is in it 



n can see it. What wonder, then, if its colour cannot 

 be seen by one to whom itself is not visible ? And 

 yet, although the cloud is not seen, it is there : and 

 so is the colour. It is, therefore, no proof of the 

 deceptiveness of the colour that it ceases to be 



