316 Prof. Challis on the Principles of Theoretical Physics. 



for the transmission of light from one point, of space to another. 

 And certainly this is a reasonable and a philosophical idea. For 

 let us consider what is the most general and patent fact in regard 

 to light that we have to account for. Some impulse, or action, 

 originates at a position in space, for instance, at a star unnum- 

 bered millions of miles distant, and after a time is felt by a 

 spectator on the earth's surface. The rate of transmission, it 

 has been ascertained, is nearly two hundred thousand miles in 

 one second. By what means, it may be asked, has the impulse 

 been transmitted from so distant a point at so rapid a rate ? To 

 say that small particles of matter are violently driven off from 

 the star in all directions, must be pronounced to be a clumsy 

 explanation compared with that suggested by Descartes, espe- 

 cially as we now know, from our acquaintance with the pheno- 

 mena of sound, that a dynamical effect may be transmitted 

 through space with great velocity by an elastic medium, without 

 the transmission of matter. I do not hesitate to say that this 

 antecedently known fact is ample justification of the hypothesis 

 that light is transmitted through space by an elastic medium 

 analogous in constitution to air. Yet this very reasonable hy- 

 pothesis meets with no favour from the mathematicians of the 

 present day. No one, as far as I am aware, except myself, 

 has endeavoured to trace the consequences of it. Both experi- 

 mentalists and theorists have not hesitated to express their dis- 

 approval of it as a hypothesis. Let them argue against it as 

 much as they please from the consequences to which it leads, 

 but to object antecedently to a hypothesis which is suggested 

 and made intelligible by ascertained facts, is, I maintain, wholly 

 contrary to right rules of philosophizing. 



It is true that a medium of a different kind has been invented 

 to account for the transmission of light through space, and for 

 other of its phenomena. According to the assumed constitution 

 of this medium, it more resembles a solid than a fluid. It is, 

 however, not exactly like any known body, a particular atomic 

 arrangement and constitution having been assigned to it expressly 

 to account for the polarization of light. The phenomena of 

 light are in this theory referred to the vibrations of discrete 

 atoms. I have from time to time given reasons for concluding 

 that there are phenomena of polarization which are incompatible 

 with such movements, and that this theory must consequently 

 be abandoned. My reasons remain unanswered. I seem, there- 

 fore, to have a right to ask that attention should be given to the 

 different course of reasoning which I have proposed, viz. that of 

 investigating mathematically the vibratory movements of a con- 

 tinuous elastic fluid, and referring to them the phenomena of 

 light. This course has required the discovery of new principles 



