Blue Colour of the Sky. 269 



appearance of light proceeding from the illuminated mass of 

 air lying between the spectator and a distant object. This 

 appearance of light is due to a slight turbidity, from which 

 the atmosphere is never quite free. Whenever there are scat- 

 tered through a transparent medium small transparent particles 

 whose density and ability to deviate the light are different 

 from those of the medium, these particles turn aside out of its 

 direct course any light which falls upon them in traversing 

 the medium, and, partly by reflection and partly by refraction, 

 ■ disperse ' it in all directions, to use an expression of optics." 

 After mentioning, as sources of turbidity in the air, dust, smoke, 

 organic matter, water-vapour on the point of condensation, 

 and the presence of hot and cold currents of air, he goes on 

 to say that, as regards the cause of turbidity in the higher 

 and drier regions of the atmosphere, which produces the blue 

 light of the sky, whether we have to do here with particles of 

 foreign matter, or whether the molecules of the air itself act 

 as a turbidity in the aether, science has no certain information 

 to give. 



" As regards, however, the colour of the light reflected by the 

 disturbing particles," he resumes, "this depends practically on 

 their size. When a log of wood floats on water, and we excite 

 small wave rings by letting a drop of water fall near it, these 

 waves are reflected by the floating log as though it were a 

 fixed wall. But in the long waves of the sea, such a log of 

 wood would be tossed up and down without the form of the 

 waves being materially altered in their progress. Now light 

 also is known to be an undulatory motion transmitted through 

 the aether that fills space. The red and yellow rays have the 

 longest waves, the violet and blue the shortest. Very small 

 bodies disturbing the continuity of the aether will therefore 

 reflect the latter rays notably more than the former. In fact, 

 the finer the disturbing particles are, the bluer the light of the 

 turbid medium ; while larger particles reflect more equally 

 light of every colour, and for this reason cause a whiter tur- 

 bidity. Of such kind is the blue of the sky, i. e. of the turbid 

 atmosphere seen against black space. The purer and more 

 transparent the air, the bluer the sky. In like manner it 

 becomes bluer and darker when we ascend a high mountain, 

 partly because the air high up is freer from turbidity, partly 

 because there is, of course, less air above our head. But the 

 same blue which we see before the dark background of space, 

 appears also before terrestrial objects, e.g. distant mountain's 

 in shadow or wooded, whenever a thick layer of illuminated 

 air lies between them and us. It is the same ' air-light ' 

 which makes both sky and mountain blue ; only in the former 



