254 FRAGMENTS OF SCIENCE. 



But there is another subject connected with our firma 

 ment, of a more subtle and recondite character than even 

 its color. I mean that &quot; mysterious and beautiful phenom 

 enon,&quot; 1 the polarization of the light of the sky. The po 

 larity of a magnet consists in its two-endedness, both ends, 

 or poles, acting in opposite ways. Polar forces, as most of 

 you know, are those in which the duality of attraction and 

 repulsion is manifested. And a kind of two-sidedness 

 noticed by Huyghens, commented on by Newton, and dis 

 covered by a French philosopher, named Malus, in a beam 

 of light which had been reflected from one of the windows 

 of the Luxembourg Palace in Paris receives the name of 

 polarization. We must now, however, attach a distinct 

 ness to the idea of a polarized beam, which its discoverers 

 were not able to attach to it. For in their day men s 

 thoughts were not sufficiently ripe, nor optical theory suffi 

 ciently advanced, to seize upon or express the physical 

 meaning of polarization. When a gun is fired, the explo 

 sion is propagated as a wave through the air. The shells 

 of air, if I may use the term, surrounding the centre of con 

 cussion, are successively thrown into motion, each shell 

 yielding up its motion to that in advance of it, and return 

 ing to its position of equilibrium. Thus, while the wave 

 travels through long distances, each individual particle of 

 air concerned in its transmission performs merely a small 

 excursion to and fro. 2 In the case of sound, the vibrations 

 of the air-particles are executed in the direction in which 

 the sound travels. They are, therefore, called longitudinal 

 vibrations. In the case of light, on the contrary, the vibra 

 tions are transversal; that is to say, the individual particles 

 of ether move to and fro across the direction in which the 

 light is propagated. In this respect waves of light resem 

 ble ordinary water-waves, more than waves of sound. In 



1 ncrschcl s Meteorology, Art. 233. 



2 Lectures on Sound, p. 3. (Longmans.) 



