52 MR. J. SMITH ON THE ORIGIN OF COLOUR 



we are called on to make, the two systems do not in 

 reality differ so widely as might be supposed, — so very 

 little, indeed, that I am warranted in thinking that the 

 arguments which will overturn the emission theoi'y will be 

 apt to sap the foundations of the wave theory ; for I do not 

 consider the latter theory to consist in the mere hypothesis 

 of a circumambient ether, but rather in the principles of 

 interpretation which it adopts in the explanation of natural 

 phenomena. For that the two systems are intimately con- 

 nected is manifest when we consider that not only has the 

 theory of the composition of light been adopted from 

 Newton, but that it was Newton who first of all ventured 

 on the calculation of the length of wave proper to each 

 element of the compound. Neither theory can indeed 

 altogether dispense with the assumption of the compound 

 nature of light. Newton's theory presupposes a difference 

 in the size of the molecules, and consequently a difference 

 in the length of the wave of each molecule, so as to account 

 for the difference in the refrangibility of the differently 

 coloured rays; the undulatory theory takes for granted 

 either an inherent difference in the original rays of the light 

 itself, and considers the ether therefore to be a compound 

 substance ; or it requires a difference in the velocity of 

 the ether within and without the refracting media, sup- 

 posing the light to be homogeneous ; which, however, leaves 

 the phenomenon of dispersion unaccounted for, and con- 

 sequently the advocates of the wave system are compelled 

 to fail back on the theory of the compound nature of light 

 as unfolded by Newton. 



75. Both theories recognise only positive terms; neither 

 admits a negative element, or any inert property in matter 

 in order to account for these phenomena. But one of the 

 strongest arguments in favour of a negative term on the 

 wave hypothesis is founded on the nature of the element 

 itself I do not mean to discuss the many difficult ques- 



