228 Prof. Tyndall on the Blue Colour of the Sky, [Jan. 14 



cipient cloud. In fact the azure revealed by such a beam would be to 

 all intents and purposes that which I have called a " blue cloud" *. 



But, as regards the polarization of the sky ,we know that not only is the 

 direction of maximum polarization at right angles to the track of the solar 

 beams, but that at certain angular distances, probably variable ones, from 

 the sun, "neutral points," or points, of no polarization exist, on both sides 

 of which the planes of atmospheric polarization are at right angles to each 

 other. 



I have made various observations upon this subject which I reserve for 

 the present; but pending the more- complete examination of the questiou 

 the following facts and observations bearing upon it are submitted to the 

 Royal Society. 



The parallel beam employed in these experiments tracked its way through 

 the laboratory air exactly as sun -beams are seen to do in the dusty air of 

 London. I have reason to believe that a great portion of the matter thus 

 floating in the laboratory air consists of organic germs, which are capable 

 of imparting a perceptibly bluish tint to the air. This air showed, though 

 far less vividly, all the effects of polarization obtained with the incipient 

 clouds. The light discharged laterally from the track of the illuminating 

 beam was polarized, though not perfectly, the direction of maximum polari- 

 zation being at right angles to the beam. 



The horizontal column of air thus illuminated was 18 feet long,, and 

 could therefore be looked at very obliquely without any disturbance from a 

 solid envelope. At all points of the beam throughout its entire length the 

 light emitted normally was in the same state of polarization. Keeping the 

 positions of the Nicol and the selenite constant, the same colours were 

 observed throughout the entire beam when the line of vision was perpen- 

 dicular to its length. 



I then placed myself near the end of the beam as it issued from the 

 electric lamp, and looking through the Nicol and selenite more and more 

 obliquely at the beam, observed the colours fading until they disappeared. 

 Augmenting the obliquity the colours appeared once more, but they were 

 now complementary to the former ones. 



Hence this beam, like the sky, exhibited its neutral point, at opposite 

 sides of which the light was polarized in planes at right angles to each other. 



Thinking that the action observed in the laboratory might be caused in 



* The opinion of Sir John Herschel, connecting the polarization and the blue colour 

 of the sky is verified by the foregoing results. " The more the subject [the polarization 

 of skylight] is considered," writes this eminent philosopher, " the more it will be found 

 beset with difficulties, and its explanation when arrived at will probably be found to carry 

 with it that of the blue colour of the sky itself and of the great quantity of light it 

 actually does send down to us." " We may observe, too," he adds, " that it is only where 

 the purity of the sky is most absolute that the polarization is developed in its highest 

 degree, and that where there is the slightest perceptible tendency to cirrus it is materially 

 impaired." This applies word for word to the " incipient clouds." 



