404 A SHORT HISTORY OF SCIENCE 



to the sun. We have seen that the sun is always opposite the rain- 

 bow ; the line between the centre of the bow and the centre of the 

 sun passing through the eye of the observer. If the sun were appar- 

 ently stationary, this would involve the bow moving much faster than 

 the observer, the latter moving through the same angle, but at less 

 distance from the apex. But this is not so. Therefore there is an 

 apparent motion of the sun concurrently with that of the bow. The 

 case is analogous to what happens when a hundred men are ranged in 

 line facing the sun. Each sees the sun in front of him. Their shadows 

 seem parallel, though we know in reality they must diverge, yet 

 owing to the vast distance of the sun this divergence is imperceptible. 

 We are thus brought to the conclusion that, supposing a rainbow to 

 occur, each of the hundred men, facing backwards, would see a dif- 

 ferent rainbow, to the centre of which his own shadow would point. 

 The rays causing the iris are therefore not incident rays, otherwise 

 the colour would appear fixed in the cloud. And for the same reason 

 they are not refracted rays, for in refraction the image does not follow 

 the change of place of the observer, as is the case here. One condi- 

 tion of the phenomenon is that the atmosphere shall be more illumi- 

 nated at the standpoint of the observer, and less at the position of the 

 cloud. The movement of the sun from east to west during the 

 appearance of the rainbow may be left out of account. 



CHAPTER VIII 



The colours in the bow arise from an ocular deception. They are 

 analogous to those which appear when the eyes are weak or half-shut. 

 They are not due to the same cause as the colours produced when 

 light shines through a crystal, since these do not, like the colours 

 of the rainbow, shift with the position of the observer. 



CHAPTER IX 



Each drop of rain in the cloud is to be regarded as a spherical mirror ; 

 these being small and close together, the effect is that of a continuous 

 image rather than of a multitude of images. The colour is due to 

 the distortion of the image caused by the sphericity of the mirror. 



CHAPTER X 



The diversity of colours has been attributed to varieties in the 

 texture of the cloud, the denser parts producing violet and blue, the 



