432 A new Explanation of the Colour of the Ski/. 



yellow impression simply dies away faster than the comple- 

 mentary blue one ; and as the general intensity decreases, the 

 ray seems bluer and bluer. 



An interesting question is that concerning the nature of the 

 medium in our atmosphere by means of which the light 

 reaching us from the open sky has been reflected. Such 

 light is polarized ; and the angle of incidence at which its po- 

 larization is most complete corresponds to that which we 

 should expect, were the refractive index of the reflecting- 

 substance =1* (absolute index =1-00029). 



A natural conclusion is that the atmosphere itself forms the 

 reflecting medium, since common air has the refractive index 

 1*000294. The common supposition seems nevertheless to be 

 that the reflexion occurs at the surfaces of particles of water 

 and of other foreign matter floating in the air. How this 

 hypothesis is to be reconciled with the condition just stated, 

 that the refractive index of the reflecting medium =1, it is 

 not easy to imagine. Clausius, in accordance with his theory, 

 supposes the light to be reflected from minute bubbles of 

 water. The atmosphere, he argues, decreasing in density by 

 imperceptible degrees upward, offers no break or discontinuity 

 at which reflexion can occur. Nothing, I submit, can be 

 further from the fact. Were our atmosphere visible, we 

 should find it to be in a constant state of agitation, broken up 

 into currents, vertical as well as horizontal, twisting and turn- 

 ing and eddying in futile efforts to reach a position of equili- 

 brium. Differences of pressure and of temperature alike serve 

 to cause this varied motion. Air warmed by contact with the 

 soil in places exposed to the sun's rays rises to make room 

 for cooler heavier masses ; and so the horizontal currents 

 which are commonly recognized as wind form but a small 

 portion of the movements constantly taking place. The effect 

 of such currents will be to form numerous surfaces at which 

 discontinuities of density and of motion will occur. At each 

 of these there will be reflexion f. 



That mist, drops of water, and other foreign substances in 

 the atmosphere also reflect light is an independent fact. The 

 presence of such light in the sky is easily shown. It is pola- 



* See Tyndall, article already cited. 



t Whenever moist air and dry come into contact and tlie boundary be- 

 tween them is sharply denned, another cause of reflexion from the clear 

 slcy occurs. Ketteler, Beobachtunyen iiber die Farbenzerstreuung der Gase, 

 Honn, 1865, shows that the refractive index of moist air is larger than 

 when the air is dry. 



