SIX LECTURES ON LIGHT. 



half of it taken from a right-handed and the 

 other from a left-handed crystal. Placing 

 the plate in front of the polarizer, we turn 

 one of the Nicols until the two halves of the 

 plate show a common puce color. This 

 yields an exceedingly sensitive means of ren- 

 dering the action of a magnet upon light 

 By turning either the polarizer or 



the analyzer through the smallest angle, tne 

 uniformity of the color disappears, and the 

 two halves of the quartz show different colors. 

 The magnet also produces this effect. The 

 puce-coicred circle is now before you on the 

 screen. (See Fig. 21 for the arrangement of 

 the experiment. N is the nozzle of the lamp, 

 H the first Nico), Q the biquartz plate, L a 

 lens, M the electro-magnet, and P the second 

 Nicol.) Exciting the magnet, one half of 

 the image becomes suddenly red, the other 

 half green. Interrupting the current, the 

 two colors fade away, and the primitive puce 

 is restored. The action, moreover, depends 

 upon the polarity of the magnet, or, in other 

 words, on the direction of the current which 

 surrounds the magnet. Reversing the cur- 

 rent, the red and green reappear, but they 

 have changed places. The red was for- 

 merly to the right, and the green to the left ; 

 the green is now to the right, and the red to 

 the left. With the most exquisite ingenuity, 

 Faraday analyzed all those actions ar.d stated 

 their laws. This experiment, however, long 

 remained rather as a scientific curiosity than as 

 a fruitful germ. That it would bear fruit of 

 the highest ; mportance, Faraday felt pro- 

 foundly convinced, and recent researches are 

 on the way to verify his conviction. 



A few words more are necessary to com- 

 plete our knowledge ot the wonderful inter- 

 action between ponderable molecules and the 

 ether interfused among them. Symmetry of 



molecular arrangement implies symmetry on 

 the part of the ether ; atomic dissymmetry, 

 on the other hand, involves the dissymmetry 

 of the ether, and, as a consequence, double 

 refraction. In a certain class of crystals the 

 structure is homogeneous, and such crystals 

 produce no double refraction. In certain 



other crystals the moJ r jles are ranged sym- 

 metrically around ? Certain line, and not 

 around others. A'ouglhe former, therefore, 

 the ray is undivided, while along ail the 

 others we have double refraction. Ice is a 

 familiar example ; it is built with perfect 

 symmetry around the perpendiculars to the 

 planes of freezing, and a ray sent through 

 ice in this direction is not doubly refracted ; 

 whereas, in all other directions, it is. Ice- 

 land spar is another example of the same 

 kind : its molecules are built symmetrically 

 round the line uniting the two blunt angles 

 of the rhomb. In this direction a ray suffers 

 no double refraction, in all others it does. 

 This direction of double refraction is called 

 the optic axis of the crystal. 



Hence, if a plate be cut from a crystal of 

 Iceland spar perpendicular to the 3x:s, a." 

 rays sent across this plate in the dirction c 

 the axis will produce but cne image. Bu* 

 the moment we deviate from the parallelism 

 with the axis, double refraction sets in. If, 

 therefore, a beam that has been rendered 

 conical by a converging lens be sent through 

 the spar so that the central ray of the cone 

 passes along the axis, this ray only will es- 

 cape double refraction. Each of the others 

 will be divided into an ordinary and extraor- 

 dinary ray, ihe one moving more slowly 

 through the crystal than the other ; the one, 

 therefore, retarded with reference to the 

 other. Here, then, we have the conditions 



