126 FRAGMENTS OF SCIENCE 



But, as regards tlie 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 polar- 

 ization 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 examina- 

 tion 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 mat- 

 ter 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 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 polarization 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 polarization. Keeping 

 the positions of the Nicol and the selenite constant, the 

 same colors were observed throughout the entire beam, 

 when the line of vision was perpendicular to its length. 



The horizontal column of air, thus illuminated, was 

 eighteen feet long, and could therefore be looked at very 

 obliquely. I placed myself near the end of the beam, as 

 it issued from the electric lamp, and^ looking through the 



