362 Intelligence and Miscellaneous Articles. 



the beam was brought to extinction by the proper adjustment of an 

 analysing nicol. A measured current of electricity was now passed 

 around the coil. This necessitated a readjustment and rotation of 

 the analysing nicol to reproduce extinction of the beam. "Within 

 the limits tried, this rotation was proportional to the current 

 strength. As a mean of many measurements, it was found that 

 a current of 1 ampere required a rotation of 78 minutes of the 

 analyser. Accordingly 278 amperes would be required to rotate 

 the plane through 360°, providing the proportionality between 

 current strength and rotation remained unaltered. 



Now, if we consider a plane-polarized ray of light to be made 

 up of two opposite circularly-polarized rays, then a particle of 

 setber in the bisulphide of carbon describes a simple harmonic 

 oscillation in a plane. This motion in a straight line is the re- 

 sultant of the two oppositely directed, equiperiodic, circular 

 rotations of equal amplitude. If now a magnetic field be created, the 

 particle undergoes an instantaneous circular electric displacement 

 which results in the retardation of one and the acceleration of the 

 other component rotation. The line of oscillation suffers rotation 

 as a result, and assumes a new position. The displacement must 

 be instantaneous, for, were it continuous, the line of oscillation 

 would continue to rotate and the analyser could not be made to 

 produce extinction. If, now, instead of allowiug the magnetic 

 field to produce this circular displacement, we superimpose, by 

 mechanical means, a third rotation upon the two existing com- 

 ponents, then a magnetic field should result and an electromotive 

 force be induced in a coil surrounding that field. Such a result 

 would be obtained by rotating the polarizing nicol. The rapidity 

 of rotation must be very great, and, if it requires 278 amperes (an 

 impressed electromotive force of 2000 volts) to rotate the plane 

 through 360°, then to produce this electromotive force the polarizer 

 must be revolved with a frequency of the same order as of the 

 oscillations of light. But a nicol cannot be revolved much above 

 200 times per second. The centrifugal force resulting from a 

 higher rate will, owing to the strain produced, interfere with the 

 performance of its functions as a polarizer. This rate of 200 

 revolutions per second would produce, in the apparatus employed, 

 an electromotive force of perhaps 0-000000001 volt, giving a 

 current too small to be detected by any galvanometer in my 

 laboratory. Hence use was made of the extreme delicacy of the 

 telephone as a substitute, and a swinging of the plane instead of a 

 revolution. 



The arrangement of apparatus was as follows : — Light from an 

 arc lamp, after passing through a large nicol, was reflected, at a 

 very obtuse angle, from a small movable mirror and then passed 

 through the bisulphide of carbon in the coil before-mentioned. 

 The two terminals of the coil were carried to a room three stories 

 below and in another part of the building. Here they were con- 



