90 ON THE RELATION OF OPTICS TO FAIN'TING 



these are repelled by the floating wood as if it. were a 

 solid wall. But in the long waves of the sea, a block 

 of wood would be rocked about without the waves 

 being thereby materially disturbed in their progress. 

 Now light is well known to be an undulatory motion 

 of the ether which fills all space. The red and yellow 

 rays have the longest waves, the blue and violet the 

 shortest. Very fine particles, therefore, which disturb 

 the uniformity of the ether, will accordingly reflect 

 the latter rays more markedly than the red and yellow 

 rays. The light of turbid media is bluer, the finer 

 are the opaque particles ; while the larger particles of 

 uniform light reflect all colours, and therefore give a 

 whitish turbidity. Of this kind is the celestial blue, 

 that is, the colour of the turbid atmosphere as seen 

 against dark cosmical space. The purer and the more 

 transparent the air, the bluer is the sky. In like man 

 ner it is bluer and darker when we ascend high moun- 

 tains, partly because the air at great heights is freer 

 from turbidity, and partly because there is less air above 

 us. But the same blue, which is seen against the dark 

 celestial space, also occurs against dark terrestrial 

 objects ; for instance, when a thick layer of illuminated 

 air is between us and masses of deeply shaded or 

 wooded hills. The same aerial light makes the sky 

 blue, as well as the mountains ; excepting that in the 

 former case it is pure, while in the latter it is mixed 



