346 



NA TURE 



[August io, 1893 



a number of particles endowed with a spinning motion 

 and reacting in consequence of that spinning motion 

 with centrifugal forces against elastic forces of the 

 surrounding medium, and let the direction of axis and of 

 spin be the same in the different cases. The former 

 jelly will transmit circularly polarised waves travelling 

 parallel to the axis of the helix with different velocities 

 according . as the helical arrangement of the displaced 

 particles in the wave does or does not agree with the 

 direction of the helical structure, the latter according 

 as the direction of the motion of the particle caused by 

 ■the wave is with or against the direction of the spin. 



This illustration accounts for the essential difference 

 'between the results of observation in the two cases. 

 The turning of the plane of polarisation produced by 

 .passing the beam through the quartz, or the solution, 

 in one direction, is undone by sending the beam back 

 again ; that produced by passing the beam through a 

 proper transparent medium placed along the lines of 

 force in the magnetic field is doubled by reversing the 

 ray after one passage, and returning the light to the side 

 at which it entered. 



To understand what takes place in each case we have 

 to consider the nature of a plane polarised ray. Ac- 

 cording to the electromagnetic theory of light the 

 disturbance in a plane polarized beam consists of an 

 electric displacement in a direction at right angles to 

 the plane of polarisation as defined in the ordinary way 

 by reflection, and a magnetic displacement at right 

 angles to the electric displacement and to the line of 

 propagation, these two actions in a direct unreflected 

 beam having the same phase. It is as yet impossible to 

 say what it is in the ether that dynamically corresponds 

 to these actions ; but there can be no doubt that they 

 are phenomena due to motion of some kind of or in 

 the luminiferous ether. In the elastic solid theory of 

 light, which must whether true or false have a certain 

 correspondence with the facts, each part of the ether 

 in a plane polarised beam was supposed to have a vibra- 

 tory motion in a straight line at right angles to the 

 direction of propagation, the direction of this line being 

 the same along the whole length of the ray. Such a 

 motion as this, it was first shown by Fresnel, may be 

 regarded as the resultant of two oppositely directed 

 circular motions, simultaneously possessed by each 

 moving part of the ether. For consider a motion which 

 would carry a particle round the circle ^4 in a given 

 time Tm the direction of the arrow, and another which 







would carry a particle round the equal circle B in the 

 same time in the opposite direction. Imagine two 

 distinct particles to move with equal uniform speeds 

 round the two circles, and suppose that both particles 

 are at the top or at the bottom of their circular orbits 

 at the same time. It is clear that at any given instant 

 both are moving up or both moving down and at the 

 same speed, while when one is moving from right to 

 left the other is moving from left to right with the 

 same speed, and vice versA. Therefore if we conceive 

 these motions united in one particle, the up and down 

 velocities will be simply added together, the right and 

 left motions will cancel one another. Thus the particle 

 will have a simple vibratory motion in a vertical line 

 in the period of the circular motions and with an 

 amplitude of twice the radii of either circle. 



Further it is to be noticed that the acceleration of 

 the particle describing this rectilinear motion is the 

 resultant of the accelerations of the particles in the 

 two circular motions and that therefore the force required 

 to maintain the simple harmonic motion is at each 



NO I 24 I, VOL. 48] 



instant the resultant of the forces which would have 

 acted on the particles in the circular motions when in 

 the corresponding positions. 



Thus the rectilinear vibration of a particle in the 

 luminiferous medium may be regarded as compounded 

 of two circular motions, and the particle as subject to 

 a force compounded of the corresponding forces ; and 

 the same may be conceived of each moving particle in 

 the wave. 



Now to conceive of the motion in a beam of circularly 

 polarised light and the relative positions of the disturbed 

 particles, let a series of particles initially in a straight 

 line along the direction of propagation be displaced into 

 a helix along that line as represented in the diagram. 

 Let these particles be projected with equal speeds in 



CO tb' 



the same direction tangential to the surface of the 

 cylinder in which the helix lies, and at right angles 

 to the axis ; and further let them be constrained ever 

 after to move with uniform speed in circles round 

 the cylindrical surface. Clearly the helical arrangement 

 vv'ill move onward along the cylinder — a circularly 

 polarised wave will be propagated. The helical arrange- 

 ment, that is the wave, is propagated in one direction 

 or the other, for a given direction of the circular motions 

 according as the helix is right or left-handed, or for the 

 same direction of the helical arrangement according 

 to the direction of the circular motion. Thus the 

 direction of propagation is unchanged if both direction 

 of helix and direction of motion is reversed. All this 

 can be easily made out from diagrams (i), (2) of the 

 figure above. Diagram (i) shows part of a left-handed 

 helical arrangement of particles ; (2) part of a right- 

 handed one. Both illustrate the arrangement of particles, 

 originally in a straight line, when disturbed by the passage 

 of a circularly polarised beam. By supposing the 

 particles to have the circular motions indicated below 

 the diagrams, the propagation of the wave can be studied. 

 It follows therefore that if two such motions co-exist 

 in the same particle both waves will travel in the same 

 direction through the medium. If they travel with 

 the same speed the resulting rectilinear motion of each 



