Flat- Wavelet Resolution. 579 



have occasion to consider the conditions which must be 

 fulfilled in nature; amongst which we shall find it neces- 

 sary to include a compliance with the equation of energy. 

 The proposed assumption is convenient, as by adopting it we 

 simplify the elementary mathematics we have to use. 



27. Light, so far as it may be regarded as light propagated 

 exclusively by a medium, is light everywhere except within 

 one or two wave-lengths of some situation where there is a 

 transfer of energy between the medium and an external 

 agent, or an agent that we may regard as external : i. e. 9 an 

 agent which generates light or absorbs it or otherwise affects 

 it. "With this understanding, light that is propagated by a 

 uniform isotropic medium, which is perfectly transparent, 

 consists of waves of alternating electromagnetic stresses propa- 

 gated in obedience to the differential equation of electro- 

 magnetic waves (see Maxwell's *' Electricity and Magnetism/ 

 Part iv. Chapter xx.), viz. : 



1 rVV 



^ Y =T?W' ^ 



where k is the speed of propagation of light of wave-frequency 

 <fc in the medium. This differential equation is also the 

 diff. equ. of dynamical waves propagated in a uniform iso- 

 tropic medium, wherever the displacements are sufficiently 

 minute to admit of their being geometrically superposed. 

 Hence events will occur in suitably chosen dynamical waves 

 which correspond to the actual events in the electromagnetic 

 waves ; and we are therefore justified in using the terminology 

 of dynamics — disturbance, energy, motion, acceleration^ 

 force, transversal, and so on — understanding these terms in 

 a sense generalized in such wise as will extend them to the 

 corresponding events in electromagnetics. This proves to be 

 a great convenience. It is even legitimate for us to fix our 

 attention upon the progress of events in a dynamical model, 

 in studying which these terms can be employed in their usual 

 dynamical acceptation, since what we are occupied upon is 

 a physical resolution* of the disturbance in the medium, and 

 every such resolution is common to all the successions of 

 events whose connexion with their physical causes is re- 

 presented by equation (9) — the fundamental differential 

 equation of the connexion which here prevails. 



28. Any complete -ft of events which that differential 

 equation can indicate as occurring in space may be called 

 one of the fields of the differential equation : and if the field 



* A resolution may be physical (that is such as may actually occur in 

 nature) or merely kineniatical. 



