SIX LECTURES ON LIGHT. 



asunder. Placing 1 the quartz between the 

 polarizer and the analyzer, you see this 

 splendid color, and, turning the analyzer in 

 front, from right to left, the other colors 

 apoear in succession. Specimens of quartz 

 have been found which require the analyzer 

 to be turned from left to right, to obtain the 

 same succession of colors. Crystals of the 

 first class are therefore called right-handed, 

 ar.d, of the second class, left handed crystals. 

 With profound sagacity, Fresne , to whose 

 genius \ve mainly owe the expansion and 

 final triumph of the undulatory theory of 

 light, reproduced mentally the mechanism of 

 these crystals, and showed their action to be 

 due to the circumstance that, in them, the 

 v/aves of ether so act upon each other as to 

 produce the condition represented by our 

 rotating pendulum. Instead of being plane 

 polarized, the light in rock crystal is circu- 

 larly polarized. Two such rays transmitted 

 along the axis of the crystal, and rotating in 



although the mixture of blue and yellow pig- 

 ments produces green, the mixture of biuc 

 and yellow lights produces white. By en- 

 larging our aperture, the two images pro- 

 duced by the spar are caused to approach 

 each other, and finally to overlap. The one 

 is now a vivid yellow, the other a vivid blue* 

 and you notice that where the colors are 

 superposed we have a pure white. (See Fig. 

 20, where N is the nozzle of the lamp, Q the 

 quartz plate, L a lens, and B the birefracting 

 spar. The two images overlap at O, and 

 produce white by their mixture.) 



This brings us to a point of our inquiries 

 which, though not capable of brilliant illus- 

 tration, is nevertheless so likely to affect pro- 

 foundly the future course of scientific thought 

 that I am unwilling to pass it over without 

 reference. I refer to the experiment which 

 Faraday, its discoverer, called the magnetiza- 

 tion of light. The arrangement for thL 

 celebrated experiment is now before you. 



FIG. 19. 



opposite directions, when brouih': to inter- 

 ference by the analyzer, are demonstrably 

 competent to produce the observed phe- 

 nomena. 



1 now abandon the analyzer, and put in its 

 place the piece of Iceland spar with which 

 we have already illustrated double refraction. 

 The two images of the carbon-points are 

 now before you. Introducing a plate of 

 quartz between the polarizer and the spar, 

 the two images glow with complementary 

 colors. Emplo)ing the image of an aperture 

 instead of that of the carbon-points, we have 

 two complementary colored circles. As the 

 analyzer is caused to rotate, the colors pass 

 through various changes ; but they are al- 

 ways complementary to each other. If the 

 one be red, the other will be green ; if the 

 one be yellow, the other will be blue. Here 

 we have it in our power to demonstrate afresh j 

 a statement made in a former lecture, that, i 



We have first our electric lamp, then n Nicot 

 prism, to polarize the beam emergeni from 

 the lamp ; then an electro-magnet, then a 

 second Nicol prism, and finally our screen. 

 At the present moment the prisms are 

 crossed, and the screen is dark. I place 

 from pole to pole of the electro magnet a 

 cylinder of a peculiar kind of glass, first 

 made by Faraday, and called Faraday's heavy 

 glass. Through this glass the b^am from 

 the polarizer now passes, being intercepted 

 by the Nicol in front. I now excite the 

 magnet, and instantly light appears upon the 

 screen. On examination, we find that, by 

 the action of the magnet upon the ether con- 

 tained within the heavy glass, the plane 01 

 vibration is caused to rotate, thus enabling 

 the light to get through the anaFyzer. 



The two classes into which quartz-crystals 

 are divided have been already mentioned. 

 In my hand I hold a compound piate, ons- 



