Jnly ^, 1889] 



NATURE 



233 



jiarls of the vibrations which are executed across the desired line 

 of vibration. So we have polarizers consisting of tourmaline slices ; 

 oblique bundles of thin glass plates ; black-glass reflectors ; and 

 Nicol prisms cut from calc-spar. About the two latter I may be 

 permitted apassing word presently. These objects polarize, i.e. turn 

 into one plane, the vibrations of light falling upon them, A rough 



mechanical illustration may here be permitted me. A long 

 india-rubber cord is passed through the open ends of a box pro- 

 vided with vertical partitions. Fig. I shows the arrangement. 

 These partitions confine the motion of the cord, and effectually 

 polarize the vibrations which I now impart to the cord by 

 shaking the end of it to and fro. If the partitions are vertical^ 



Fig. I. — Box with partitions to illustrate polarization of vibrations. 



the box polarizes, into vertical vibrations only, the miscellaneous 

 vibrations which are sent to it. If rotated until its partitions are 

 horizontal, it polarizes the vibrations into a horizontal position. 



Let us now turn to the optical analogue of this experiment. 

 The large Nicol prism which I introduce into the field of the 

 electric-light lantern, polarizes the light, so that the vibrations 

 are executed simply in an up-and-down direction. Your eye 



will not detect this, the motions being millions of times too- 

 rapid. To detect the direction, an analyzer is necessary. For 

 this purpose a second apparatus of the same sort is used, for 

 then, by crossing the positions of the two, the whole of the light 

 is cut off; the second Nicol prism, if set so as to transmit only 

 horizontal vibrations, cutting off" the vertical vibrations that are 

 sent through the first prism. So, whilst the first prism serves as 



Fig. 2. — Acoustic model illustrating polarization of vibrations, p, the polarizer ; a, the analyzer. 



a polarizer, the second serves as an analyzer to detect by cutting 

 them off when turned to the proper position, the direction of the 

 polarization which had been previously impressed by the first 

 prism. 



Here I may illustrate the action of the analyzer for determin- 

 ing the plane of polarization of the vibrations, by the extinction 

 which it produces when turned to the crossed position. For this 



purpose I have refined upon the box with partitions, using in- 

 stead parallel plates of glass mounted in wooden cylinders, whilst 

 for the cord swung by hand I am using Prof. Schwedoff"'s 

 device, and am producing the vibrations in this silken cord by 

 means of an electrically-driven tuning-fork (Fig. 2). At the first 

 nodal point of the stretched cord a pair of parallel glass plates 

 acts as a polarizer, the cord from that point vibrating in the 



Fig. 3.— Vibrations cut off by turning the analyzer a at right angles to the polarizer v. 



plane thus imposed upon it. I can alter this plane at will by 

 rotating the polarizer. This polarizer, P, consisting of a pair of 



de Lepinay. In Fig. 2 the polarizer and analyzer are parallel. 

 ..ex. ..u. pu.u...... r, v..u..oc..,g .. ^i.... ^. ..You see (Fig. 3) how the vibration is extinguished when the 



glass plates, 'is mounted in a cylindrical mount, and is provided positions of analyzer and polarizer are crossed. Halt a degree ot. 

 with an arrow to indicate their direction. If now at any sub- error in the position of the analyzer produces something less than 

 sequent node I introduce a second such device, it will act as perfect extinction of the vibrations Hence it is possible, by tnis 

 an analyzer, A. This excellent suggestion is due to M. Mace | analyzer, to determine the plane of the vibrations to the accuracy 



