118 FRAGMENTS 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 polarisation j 

 exist, on both sides of which the planes of atmospheric 

 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 com- 

 plete examination of the question, the following 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 

 polarisation. Keeping the positions of the Nicol and 

 the selenite constant, the same colours were observed 



* The more the subject [the polarisation 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 polarisation 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 our ' incipient clouds.' 



