438 Prof. Powell on the Theory of the Aberration of Light, 



"ten thousand leagues away 



Into the devious air " ? 



or is such an idea so wholly improbable and unsupported 

 by analogy that all consideration of it may be discarded, and 

 the question rather put, What should lead us to imagine such 

 an exception? ov the quus probandi be thrown on the side 

 of those who would assert it ? 



On this question I would only observe that every inductive 

 law is essentially open to exceptions which may call for some 

 modification of it, but any such exceptions must be substan- 

 tiated on the most unequivocal evidence, before they can be 

 admitted as influencing the universality of the law. If this be 

 accepted as a fair rule of inductive logic, it will follow that we 

 need not trouble ourselves with imagined or possible excep- 

 tions to the general law of rectilinear propagation, which may 

 conceivably be occasioned by causes wholly unknown. If in- 

 deed we had any reasonable ground for suspecting that any 

 known cause, such as e. g. terrestrial magnetism or electricity 

 or heat, might act upon light as it approached the earth, then 

 it would be a perfectly reasonable demand to see whether those 

 causes produced any deviation, before we assumed the per- 

 fectly rectilinear course of the rays. 



But without insisting on such considerations as those just 

 referred to, it will I think be agreed on all hands, that, admitting 

 the aberration to be wholly accounted for by the known facts 

 relative to the motion of light and the earth, still the explana- 

 tion cannot be called a perfect or philosophical one until those 

 facts themselves are completely explained by a theory con- 

 necting them, and consequently the aberration, with the whole 

 assemblage of laws and phaenomena of light: and therefore 

 that, an appeal to the undulatory (or whatever is the best- 

 established) theory is in this sense a necessary part of the inves- 

 tigation. Yet we should bear in mind that it is not more 

 peculiarly essential in this case than in many others; such as 

 e.g. the case of the rainbow before adverted to. 



As another parallel case we might refer to the polarized 

 rings. Granting ihe facts of polarization, and of the interfe- 

 rences of polarized light, the formation of the rings is completely 

 explained without reference to any theory of the nature of 

 light. It is a question for the credit of any theory, whether it 

 will account for the laws of polarization and the interferences 

 of polarized light: if it does so, it explains the rings; and the 

 undulatory theory alone has been shown to do so. At the 

 same time had we no theory explaining and connecting the 

 laws of polarization and interference, we should certainly feel 

 our views of the whole subject very unsatisfactory. 



