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LXIII. Explanations of Phenomena of Light on the Hypothesis of 

 Undulations. By Professor Challis, M.A.,F.R.S., FJl.A.S.* 



THE explanations I am about to propose rest on the hypo- 

 thesis that the phenomena of light are visible effects of 

 the motions and pressures of a continuous elastic fluid the pres- 

 sure of which is proportional to the density. This hypothesis 

 brings the facts to be explained into immediate connexion with 

 the hydrodynamical theorems of which I have given demonstra- 

 tions in the Numbers of the Philosophical Magazine for June, 

 August, and October. The references to" the theorems will be 

 made by means of the articles, numbered from 1 to 48, which 

 make up the three communications. In the instances of several 

 of the more common phenomena, the theoretical explanations 

 are so obvious that little more is required than merely referring 

 to the articles. With respect to others, it will be necessary to 

 introduce some special considerations. It is, however, to be 

 understood that, whether the explanations be obvious or not, the 

 truth of the hydrodynamical theorems rests on principles and 

 reasoning altogether independent of this application of them, 

 and that they receive no additional confirmation from the success 

 with which they explain phenomena, which is only to be taken 

 as evidence of the reality of the sethereal medium and of its 

 assumed properties. I shall at present confine myself to those 

 phenomena which have no particular relations to visible and 

 tangible substances, but depend only on the qualities of the 

 medium in which the light is generated and transmitted. The 

 phenomena of reflexion, refraction, diffraction, &c. are reserved 

 for consideration after the explanations of the other class of 

 phenomena have tested the reality of the hypothetical aether and 

 of its supposed qualities. 



One of the most observable and general laws of light is its 

 transmission through space in straight lines independently of the 

 mode of its generation. This fact is theoretically explained by 

 the rectilinear axes of free motion of the aether, and by the cir- 

 cumstance that the motion resulting from any kind of disturb- 

 ance is, to the first power of the velocity, composed generally of 

 vibratory motions relative to such axes. The theorems on which 

 this explanation rests are discussed in articles II, 20-30, 34, 

 and 38. 



The law of rectilinear axes of free motion having been first 

 deduced from the assumed properties of the aether, the mathe- 

 matical reasoning then conducted to specific analytical expres- 

 sions for the motions and condensations relative to these axes, 



* Communicated by the Author. 



