LIGHT. 117 



in common, that they cannot generally be detected by the unas- 

 sisted eye; and, consequently, special instrumental means are 

 required for their investigation. The phenomena which are among 

 the most splendid in the entire range of optics must be witnessed to 

 be appreciated or understood ; and the instruments described below 

 constitute the means for rendering them visible. The explanation 

 of the facts is to be sought in the wave theory of light, by means 

 of which not only have many very diversified phenomena been 

 connected together, but in some cases results have been actually 

 predicted. According to this theory light is due to the vibrations 

 of an elastic medium called ether, which is supposed to pervade 

 all space. The vibrations take place in plane's perpendicular to 

 the direction of the ray; and the paths or orbits are straight lines, 

 or circles or ellipses. In a ray of common light the orbits of an 

 ether particle are subject to incessant and irregular changes in form 

 and position, but in a polarised ray all the orbits are similar and 

 similarly situated : and the process of polarisation is understood 

 to consist in bringing all the orbits into similar positions. 



For the exemplification of this theoretical side of the subject 

 various forms of instruments have been devised, the most com- 

 prehensive of which is known as the wave machine of Wheatstone. 

 The object of this instrument is to exhibit the results of the com- 

 bination of various kinds of vibrations meeting at various phases 

 of their motion. Another form of apparatus for illustrating wave 

 motion is exhibited by Mr. Woodward, of Birmingham. 



For plane polarisation of light three methods are in general use 

 reflexion from glass or other non-metallic substance, refraction 

 through a plate, or, better still, through a bundle of glass plates, 

 and double refraction by crystalline media. The same methods 

 serve for analysing light so polarised. Instruments have been 

 devised upon the bases of all these methods. In the polariscope 

 of Norrenberg provision is made for the application of the first 

 two methods ; in others the third method is used. The crystals 

 generally employed are tourmalines and Iceland spar. Tourma- 



