Phenomena of Wave-motion by means of Flexible Cords, 265 



perposed and produce a wave of double amplitude. One half of 

 the double cord may be tightened until its wave reaches the single 

 cord half a wave's length before the wave on the other half, 

 when interference occurs, there being little or no lateral motion 

 to be observed in the single cord. 



9. Plane of Waves. — In the experiments previously described 

 the waves were transmitted in a vertical plane ; but by properly 

 directing the motion of the hand, the waves may be transmitted 

 in planes variously inclined to the board, or in a plane parallel 

 with it. Waves in space of three dimensions, corresponding to 

 circularly polarized light, are produced by rapidly and regularly 

 moving the hand in a circle, the cord then taking the form 

 shown at the right of figs. 7 and 9. 



10. Polarization, — A series of flat boards are used as guides, 

 which are clamped on the long board. These are shown in 

 figs. 7, 8, 9. The vertical and oblique guides are each in two 

 pieces, which are so approximated to each other as to just allow 

 the cord to move freely between them. The horizontal guide is 

 in one piece only. The vertical and horizontal guides being 

 fixed as shown in figs. 7 and 8, waves in a vertical plane are 

 transmitted from that end of the rope nearest the vertical guides ; 

 the waves then pass freely through the vertical guides., but are 

 completely stopped by the horizontal one. Waves in a hori- 

 zontal plane transmitted from the other end of the apparatus 

 pass the horizontal guide, but are stopped by the vertical ones 

 (fig. 8). Waves in an oblique plane transmitted from either end 

 are resolved by the nearest guide into a component in its own 

 plane and a component at right angles which is suppressed; 

 the former passes on and is stopped by the next guide. Circularly 

 polarized waves on reaching the guides are similarly resolved 



11. Depolarization. — A pair of oblique guides are required in 

 addition to those described in 10. The arrangement of these is 

 shown in fig. 9, which needs no further explanation. The waves 

 are supposed to proceed from right to left. With a single cord 

 as in fig. 9, or with a partly double one as in fig. 6, an endless 

 variation of experiments relating to polarization maybe produced. 



12. Radiation and Absorption. — A rod of iron about 2 feet 

 in length, having an eye at the centre and at each end, is fixed 

 by means of a screw or pin through the central eye to an up- 

 right support of wood clamped at about the middle of the board 

 (fig. 10). The iron rod must be able to rotate freely about the 

 pin in a vertical plane parallel to the board, but in no other 

 plane. Attaching a cord to one end of the iron rod and conti- 

 nuing it to the end of the board, a series of properly timed waves 

 are sent along it, when the rod vibrates in synchronism with the 



