ETOER. 



which it capable of assuming negative as well as positive 

 be the nature of the process, if it is capable of being 

 by an equation of this form, the process going on at a fixed point 



ribratiom; the constant A is called the amplitude; the time - is 

 the period; and nt-px+a is the phase. 

 The configuration at a given instant is called a wave, and the distance 



the MMt-fayft. The velocity of propagation is -. When we contem- 

 plate the different parts of the medium as going through the same process in 

 oeceaaion, we nee the word undulatory to denote this character of the process 

 without in any way restricting its physical nature. 



A further insight into the physical nature of the process is obtained from 

 the fact that if the two rays are polarized, and if the plane of polarization 

 of one of them be made to turn round the axis of the ray, then when the 

 two planes of polarization are parallel the phenomena of interference appear as 

 above doeffribfld. As the plane turns round, the dark and light bands become 

 less distinct, and when the planes of polarization are at right angles, the 

 illumination of the screen becomes uniform, and no trace of interference can be 

 discovered. 



Hence the physical process involved in the propagation of light must not 

 only be a directed quantity or vector capable of having its direction reversed, 

 but this vector must be at right angles to the ray, and either in the plane 

 of polarization or perpendicular to it. Fresnel supposed it to be a displace- 

 ment of the medium perpendicular to the plane of polarization. Maccullagh and 

 Neumann supposed it to be a displacement in the plane of polarization. The 

 comparison of these two theories must be deferred till we come to the phenomena 

 of dense media. 



The process may, however, be an electromagnetic one, and as in this case 

 the electric displacement and the magnetic disturbance are perpendicular to each 

 other, either of these may be supposed to be in the plane of polarization. 



All that has been said with respect to the radiations which affect our 

 eyes, and which we call light, applies also to those radiations which do not 

 produce a luminous impression on our eyes, for the phenomena of interference 



