352 F. E. Wright — Measurement of Extinction Angles. 



decreases, and vice versa. At the moment the kinetic energy 

 of the particle becomes zero, the total energy is potential, and 

 similarly, when the kinetic energy attains its maximum, the 

 potential energy is zero. In other words, the average poten- 

 tial energy is equal to the average kinetic energy, and the 

 whole energy is twice the average kinetic energy, given in the 

 above expression. 



A measure for the intensity of light is the energy per unit 

 volume of the vibrating ether, and if in the above expression 

 for the kinetic energy m stands for the mass per unit volume, 

 the whole energy or intensity will be measured by the 

 expression, 



_ 2m7rV 



In practice, only relative intensities are encountered. The 

 relative intensities of two light vibrations of equal period (T) 

 at a given point will be in the ratio of the square of their 

 amplitudes : 



I, _ 2???7rX 2 : 2?927r 2 a 2 2 _ a* 



X ~ ~ T 2 "*" ~^ ~~ < 



In other words, the intensity of light of a given period of 



vibration (color) varies as the square of its amplitude (a) of 



vibration. 



This relation will now be made use in determining the rela- 

 tive intensities of the plane polarized light waves which 

 emerge from the upper nicol of the microscope, after having 

 passed through the lower nicol and an intervening crystal 

 plate in different positions. 



Disturbances in the ether which produce light phenomena 

 are ascribed to the action of forces on the ether mass, and if 

 two or more separate disturbances are simultaneously impressed 

 on the same element, the resultant disturbance can be calcu- 

 lated according to the principle of the resolution of forces on 

 the assumption of direct superposition of the forces. If, in the 

 case of plane polarized light, two separate vibrations be 

 imposed simultaneously on an element, the resultant vibra- 

 tion will also be in that plane, and its amplitude, on the prin- 

 ciple of superposition, is the algebraic sum of the amplitudes 

 of the components. The mathematical expression for the 

 resultant vibration of a particle simultaneously impressed by 

 two periodic disturbances of the same period but differing in 

 phase and amplitude, can be deduced from the equations of 

 the separate vibrations. 



Bli\.2v(t— t.) , . 2ir(l—t 9 ) 



y 1 = a J ^ ^ and y 2 = « 2 sin ^--JL 





