118 FKAGMENTS OF SCIENCE. 



But, as regards the polarisation of the sky, we know 

 that not only is the direction of maximum polarisation 

 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 polarisa- 

 tion, exist, on both sides of which the planes of at- 

 mospheric polarisation are at right angles to each other. 

 I have made various observations upon this subject 

 which are reserved for the present; but, pending the 

 more complete examination of the question, the follow- 

 ing facts bearing upon it may be submitted. 



The parallel beam employed in these experiments 

 tracked its way through the laboratory air, exactly as 

 sunbeams 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 

 particles, which are capable of imparting a perceptibly 

 bluish tint to the air. These also showed, though far 

 less vividly, all the effects of polarisation obtained with 

 the incipient clouds. The light discharged laterally 

 from the track of the illuminating beam was polarised, 

 though not perfectly, the direction of maximum polar- 

 isation being at right angles to the beam. At all points 

 of the beam,- moreover, throughout its entire length, 

 the light emitted normally was in the same state of 



tion and the blue colour of the sky, is verified by the foregoing 

 results. ' The more the subject [the polarisation of sky light] 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 polari- 

 sation is developed in its highest degree, and that where there is 

 the slightest perceptible tendency to cirrus it is materially im- 

 paired.' This applies word for word to our ' incipient clouds.' 



