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 " mysterious and beautiful phenom- 

 enon," ' 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 Herschcl's Meteorology, Art. 233. 



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



