COLOURS OF THIN PLATES. 149 



tint when the light is indirect ; and the colours rise in the 

 series, when the difference of the refractive densities is lessened 

 by adding salt to the water. The interval of retardation, 

 in this case, depends on the . magnitude of the transparent 

 particle. 



(163) In concluding this review of the two theories, in 

 their application to the laws of unpolarized light, it should 

 be observed that any well-imagined hypothesis may be 

 accommodated to phenomena, and seem to explain them, if 

 only we increase the number of its postulates, so as to 

 embrace each new class of phenomena as it arises. In a 

 certain sense such an explanation may be said to be true, so 

 long as it is thus made to represent all known facts ; but it 

 is no longer a theory, of whose very essence it is to ascend in 

 simplicity, at the same time that it rises in generality : 

 it is " a mob" of hypothetical laws, without connexion, order, 

 or dependence. 



These remarks apply to the explanation of the pheno- 

 mena of thin plates in the theory of emission. These 

 phenomena, Newton saw, could not be accounted for on 

 the bare hypothesis of molecules shot from the luminous 

 body, and subjected to the attractive and repulsive forces of 

 the bodies which they met in their progress. He was com- 

 pelled to add a new property to light, to endow the mo- 

 lecules with dispositions which seemed wholly alien to their 

 other properties, and which could only be connected with, 

 them by assuming a material mechanism much more com- 

 plicated than was at first proposed. But this was not all. 

 Each of the laws of thin plates was found to require a new 

 property in the fits to which they were referred ; and none 

 of these properties were in any way related to the rest, 

 or to the mechanism on which they were supposed to de- 

 pend. 



