SIX LECTURES ON LIGHT. 



on the principles of the wave theory, and he 

 succeeded in doirg so. He made highly im- 

 portant observations on the distinctive charac- 

 ter of the two beams transmitted by the spar. 

 Newton, reflecting on the observations of 

 Huyghens, came to the conclusion that each 

 of the beams had two sides ; and from the 

 analogy of this two sidedness with the two 

 tndedness of a magnet, wherein consists its 

 polarity, the two beams came subsequently to 

 be described as polarized. 



We shall study this subject of \b& polariza- 

 tion of light with great ease and profit by 

 means of a crystal of tourmaline. But let us 

 start with a clear conception of an ordinary 

 beam of light. It has been already explained 

 that the vibrations of the individual ether- 

 particles are executed across the line of prop- 

 agation. In the case of ordinary light we 

 are to figure the ether particles as vibrating 

 in all directions, or azimuths, as it is some- 

 times expressed, across this line. 



Now, in a plate of tourmaline cut parallel 

 to the axis of the crystal, the beam of incident 

 light is divided into two, the one vibrating 

 parallel to the axis of the crystal, the other at 

 right angles to the axis. The grouping of 

 the molecules, and of the ether associated 

 with the molecules, reduces all the vibrations 

 incident upon the crystal to these two direc- 

 tions. One of these beams, namely that one 

 whose vibrations are perpendicular to the 

 axis, is quenched with exceeding rapidity by 

 the tourma ine, so '.hat, after having passed 

 through a very small thickness of the crystal, 

 the light emerges with all its vibrations re- 

 duced to a single plane. In this condition it 

 is what we call a beam of plane polarized 

 light. 



A moment's reflection will show, if what 

 has been stated be correct, that, on placing 

 a second plate of tourmaline with its axis 

 parallel to the first, the light will pass through 

 both ; but that, if the axes be crossed, the 



FIG. 10. 



light that passes through the one plate will 

 be quenched by the other, a total interception 



of the light being the consequence. The 

 image of a plate of tourmaline, 1 1 (Fig. 10), 

 is now before you. I place parallel to it 

 another plate, t' t f : the green of the crystal 

 is a little deepened, nothing more. By means 

 of an endless screw, I now turn one of the 

 crystals gradually round ; as long as the two 

 plates are 'oblique to each other, a certain 

 portion of light gets through ; but, when they 

 are at right angles to each other, the space 

 common to both is a space of darkness, as 

 shown in Fig. n. 



Let us return to a single plate ; and let me 

 say that it is on the green light transmitted 

 by the tourmaline that you are to fix your at- 

 tention. We have now to illustrate the two- 

 sidedness of that green light. The light sur- 

 rounding the green image being ordinary 

 light, is reflected by a plane glass mirror in i 

 all directions ; the green light, on the con- 

 trary, is not so reflected. The image of the 

 tourmaline is now horizontal ; reflected up- 

 wards, it is still green ; reflected sideways, 

 the image is reduced to blackness, because of 

 the incompetency of the green 1'ght to be re- 

 flected in this direction. Making the plate 

 of tourmaline vertical and reflecting it as 

 before, in the upper image the light is 

 quenched ; in the side image you have now 

 the green. Picture the thing clearly. In 

 the one case the mirror receives the impact 

 ot the edges of the waves, ana the green light 

 is quenched. In the other case the sides of 

 the waves strike the mirror, and t e green 

 light is reflected. To render the extinction 

 complete, the light must be received upon 

 the mirror at a special angle. What this 

 angle is we shall learn presently. 



The quality of two-sidedness conferred 

 upon light by crystals may also be conferred 

 upon it by ordinary reflection. Malus made 

 this discovery in 1808, while looking through 

 Iceland spar at the light of the sun reflected 

 from the windows of the Luxembourg palace 

 in Paris. I receive upon a plate of window- 

 glass the beam from our lamp ; a great por- 

 tion of the light reflected from the glass is 

 polarized ; the vibrations of this reflected 

 beam are executed, for the most part, paral- 

 lel to the surface of the glass, and, if the 

 glass be held so that the beam shall make an 

 angle of 58 with the perpendicular to the 

 glass, the whole of the reflected beam is polar- 

 ized. It was at this angle that the image 

 of the tourmaline was completely quenched 

 in our former experiments. It is called the 

 polarizing angle. 



And now let us try to make substantially 

 the experiment of Malus. I receive the beam 

 from the lamp upon this plate of glass and 

 reflect it through the spar. Instead of two 

 images, you see but one. So that the light, 

 when polarized, as it now is, can only get 

 through the spar in one direction, and conse- 

 quently produce <but one image. Why is 

 this ? In the Iceland spar, as in the tour- 

 maline, all the vibrations of the ordinary light 



