LIGHT. 1 35 



extreme ; and if we moreover suppose its tension 

 to be very great, which the vast velocity of light 

 requires us to suppose, the vibrations by which 

 light is propagated will be transverse vibrations, 

 that is the motion to and fro will be athwart the 

 line along which the undulation travels. The 

 reader may perhaps aid his conception of this 

 motion, by attending to the undulation of a long 

 pendant streaming in the wind from the mast- 

 head of a ship : he will see that while the undu- 

 lation runs visibly along the strip of cloth, from 

 the mast-head to the loose end, every part of the 

 strip in succession moves to and fro across this 

 line. 



From this transverse character in the lumi- 

 niferous vibrations, all the laws of polarization 

 necessarily follow : and the properties of trans- 

 verse vibrations, combined with the properties of 

 vibrations in general, give rise to all the curious 

 and numerous phenomena of colours of which we 

 have spoken. If the vibrations be transverse, 

 they may be resolved into two different planes ; 

 this is polarization : if they fall on a medium 

 which has different elasticity in different direc- 

 tions, they will be divided into two sets of vibra- 

 tions ; this is double refraction ; and so on. Some 

 of the new properties, however, as the fringes 

 of shadows and the colours of thin plates, follow 

 from the undulatory theory, whether the vibra- 

 tions be transverse or not. 



It would appear, therefore, that the propaga- 



