276 Professor Powell on the Nature of the 



ever, had been framed long before there was a sufficient accu- 

 mulation of facts, or even the means of reducing a large portion 

 of those facts which were collected to any regular law. Each 

 of the rival theories, — those of emission and of undulations, was 

 ori^naliy framed upon a very scanty stock of phenomena, and 

 devised with no view beyond the explanation of that limited ex- 

 tent of results. Hence it will hardly be a matter of surprise 

 that either hypothesis, in its crude form, should soon have been 

 found to apply very ill to a number of new phenomena as they 

 were successively discovered ; nor that, in proportion as the de- 

 ficiencies of each were manifest to the more sober inquirers after 

 philosophic truth, their respective partisans should become more 

 vehement in upholding them ; each charging upon the other de- 

 fects, which indeed could not be denied, whilst blind to the im- 

 perfections of their own hypothesis. Such controversies have 

 not diminished in warmth in the present times. A vast range 

 of new phenomena has been disclosed, which has led to the ne- 

 cessity of more extended and complex theories for their expla- 

 nation; and, unquestionably, that hypothesis which has attracted 

 most general attention, and on which the most profound mathe- 

 matical skill has been expended, is the system of undulations 

 propounded by Huygens, and modified and enlarged by the in- 

 vestigations of Young and Fresnel. 



Much has been said of late both for and against the claims 

 of the undulatory theory ; and, perhaps, much on both sides 

 which has evinced but imperfect conceptions of the real preten- 

 sions of that theory, or of the real nature of the evidence on 

 which it has claimed acceptance. Whilst, on the one hand, it 

 has been sometimes held up as possessing all the characteristics 

 of certainty ; it has, on the other, been as strongly and unreser- 

 vedly condemned, because its principles have failed to explain 

 some well established and some alleged facts. On the one 

 hand, the language of its advocates has sometimes represented 

 its explanation of the phenomena as equivalent to demonstra- 

 tion, and the existence and properties of the etherial fluid as 

 physical facts ; on the other, certain close comparisons of cal- 

 culated and observed phenomena, certain difficult cases, for the 

 solution of which the formulas have not been rendered available, 

 certain real or apparent anomalies, and lastly, and principally, 



