PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 473 



be supposed always horizontal. So long as the edges E F and G H are 

 directed north-south the polarized beam will be reflected from the 

 upper glass plate, even when the beam falls upon it at the polarizing 

 angle. When, however, the upper plate forms an angle of 32 with 

 the beam, and is so turned that 

 its upper and lower edges, E F and 

 G H, are directed east-west, it abso- 

 lutely refuses to reflect the polar- 

 ized beam. On turning the upper 

 plate round o o' as an axis, while 

 its inclination to o o' is always 32, 

 there are two positions in which 

 the reflected light is at its maxi- 

 mum, namely, E FG H and E" F" G" H", 

 and two at which it is totally 

 abolished, namely, E' F' G' H' and 

 E'" F'" G'" H"'. At intermediate 

 positions the amount of light re- 

 flected varies from one to the other 

 extreme. 



The experiments of Arago and 

 Fresnel were designed to test the 

 properties of polarized light as 

 regards interference. We have 

 already seen the genius of Fresnel 

 in the application of the undula- 

 tory theory to the facts of diffrac- 

 tion and interference phenomena 

 (pages 45 5 and 464). We select 

 for description an experiment con- 

 trived by Arago, because it is 

 the most direct and convincing. 

 Two very fine parallel slits were 

 made in a thin sheet of copper 

 close to each other. A bundle 

 of fifteen thin laminae of mica was 

 cut in two, and of these it is 

 obvious that in the parts which 

 were contiguous before separation the thickness must have been very 

 nearly the same. These bundles polarized almost completely light 

 which fell upon them at an angle of 30 to the surface, and one of them 

 was placed at this angle of inclination before each of the slits in the 

 copper plate, these slits being illuminated by light from a point or 

 single slit. When the two bundles of plates were inclined in the same 

 direction, the bands formed by the interference of the polarized rays 

 were distinctly seen, exactly as when two rays of ordinary light act 



FIG. 215. 



