vii PRISMATIC EFFECTS 31 



plain that no colour is formed in the rod, but only 

 a false appearance of colour, such as I mentioned 

 above, which the neck of a pigeon, as it is bent 

 hither and thither, alternately puts on and off. 

 This, I say, is seen likewise in the case of a mirror, 

 which assumes no real colour, but only a certain 

 imitation of the colour of a foreign body. 



Still, this one point requires explanation ; it is 3 

 not the sun's image that is beheld in that glass 

 stick, because it is not capable of expressing it 

 accurately. True enough it tries to reproduce the 

 image, because the material is smooth and suitable 

 for this purpose. But it fails because its shape is 

 unsymmetrical. If it had been suitably constructed, 

 it would reflect as many images of the sun as it 

 had faces. But since the sides are not distinctly 

 separated from each other, and not bright enough 

 to serve as mirrors, the images are only incipient, 

 not fully expressed ; they get confused through 

 being crowded together, and are reduced to the 

 appearance of a single band of colour. 



VIII 



BUT to return why does the bow not complete 

 the full circle in its form, but appear as only a semi- 

 circle when stretched to the full extent of its 

 greatest span ? Some are of opinion that the 

 reason is that the sun, being much higher than the 

 clouds, strikes them only on the upper side. Hence 

 their lower parts are not touched by his light. 

 Receiving the sun only on one side, the clouds 

 reproduce only one portion of him, and this is 

 never more than a half. There is very little force 



