26 



SIX LECTURES ON LIGHT. 



are reduced to two planes at right angles to 

 each other ; but, unlike the tourmaline, both 

 beams are transmitted with equal facility by 

 the spar. The two beams, in short, emerg- 

 ent from the spar are polarized, their direc- 

 tions of vibration being at right angles to 

 each other. When, therefore, the light was 

 Dolarized by reflection, the direction of vibra- 

 tion in the spar which corresponded to the 



conclude? That the green light will be 

 transmitted along the latter, which is parallel 

 to the tourmaline, a:id not along the former, 

 which is perpendicular to it. Hence we may 

 infer that one image of the tourmaline will 

 show the ordinary green light cf the crystal, 

 while the other image will be black. Let us 

 test our reasoning by experiment : it is veri- 

 fied to the letter. (Fig. 13.) 



FIG 



direction of vibration of the pokrized beam 

 transmitted it, and that direction only. But 

 one image, therefore, was possible under the 

 conditions. 



And now you have it in your power to 

 check many of my statements, and you will 

 observe that such logic as connect our experi- 

 ments is simply a transcript of the logic of 

 Nature. On the screen before you arc the 



Let us push our test still further. By 

 means of an endless screw, the crystal can 

 l*e turned ninety degrees round. The black 

 image, as I turn, becomes gradually brighter 

 and the bright one gradually darker; at an 

 angle cf forty- five degrees both images are 

 equally bright (Fig. 13); while, when ninety 

 degrees have been obtained, the axis cf the 

 crystal being then vertical, the briglu and 



FIG 



two disks of light produced by the double re- 

 fraction of the spar. They are, as you 

 know, two images of the aperture through 

 which the light issues from the camera. 

 Placing the tourmaline in front of the aper- 

 ture, two images of the crystal will be ob- 

 tained ; but now let us reason out what is to 

 be expected from this experiment. The light 

 emergent from the tourmaline is polarized. 



black ftnages have changed places. (Fig. 

 I4-) 



Given two beams transmitted through Ice- 

 land spar, it is perfectly manifest that we 

 have it in our power to determine instantly, 

 by means of a plate of tourmaline, the direc- 

 tions in which the ether-particles vibrate in 

 the two beams. I might place the double- 

 refracting spar in any position whatever. A 



FIG. 14. 



Placing the crystal with its axis horizontal, 

 the vibrations of the transmitteu light will be 

 horizontal. Now the spar, as already stated, 

 has two perpendicular directions ot vibration, 

 one of which, at the present moment, is ver- 

 tical, the other horizontal. "What are we to 



minute's trial with the tourmaline would 

 enable you to determine the position which 

 yields a black and a bright image, and from 

 these you would at once infer the directions 

 of vibration. 



Further, the two beams from the spar 



