472 HISTORY OF SCIENCE. 



tourmaline is viewed by reflection from the glass, it will be seen of its 

 true colour when held vertically, but held horizontally it will appear 

 as opaque as a piece of metal. If we examine the light which passes 

 through the plate of glass, it will be seen through the tourmaline in 

 all positions, but brightest when the axis is horizontal, darkest when 

 it is vertical. This shows that some of the light is polarized, and, be 

 it particularly observed, this polarization is in & plane perpendicular to 

 that in which the reflected beam is polarized. By placing several plates 

 of glass in contact with the first we can, as already observed, obtain 

 the polarization of practically the whole of the transmitted light. The 

 reader will presently see the use made of these facts by Fresnel in an 

 experiment which may be termed crucial, for it decided the adoption 



FIG. 214. 



of transverse vibration into the theory of light. Models have been 

 constructed to illustrate the perpendicularity of the two planes of polari- 

 zation in the refracted and transmitted beams. In Fig. 214 B may be 

 supposed (with the differences already pointed out) to represent a 

 beam of ordinary light incident upon a bundle of upright glass plates. 

 The shaded vertical plane c indicates the direction of the vibrations 

 in the reflected beam ; the unshaded plane D, in which the vibrations 

 are horizontal, represents the transmitted beam. Before describing 

 those experiments of Fresnel and Arago which determined the adoption 

 of the theory of transverse vibrations, we place before the reader a 

 diagram (Fig. 215) to further illustrate the application of that theory 

 to the discovery of Malus. Let A B c D be a rectangular plate of glass, 

 and so a beam falling upon it at the angle of 32. We shall suppose 

 the edges A B and c D to be horizontal, and directed north-south, and 

 the plane of incidence i.e., the plane containing the incident and re- 

 flected rays to be east-west, the reflected ray o o' being vertical. The 

 vibrations in the reflected ray will then be confined to a vertical plane 

 directed north-south, that is, the motions of the ethereal particles, as 

 shown by the small arrows, will now be parallel to the surface of the 

 glass. Let this reflected polarized ray be received upon another rect- 

 angular plate of glass E F G H, the edges of which, E F and G H, must 



