RESIDUAL PHENOMENA. 101 



RESIDUAL PHENOMENA. 



Br Peofessoe PATTISON MUIE. 



TN his "Preliminary Discourse on the Study of Natural Philosophy," 

 J- Sir John Herschel remarks upon the importance of examining those 

 phenomena of nature which are not wholly explicable in terms of anv 

 well-established theory. Instances of such residual phenomena, as Sir 

 John Herschel terms them, are given in the discourse. 



Newton's theory of comets, viz., that these bodies obey the law of 

 gravitation while revolving in oblique orbits round the sun, appeared 

 to account for the facts which had been noticed concerning the comet 

 of Halley ; but the period calculated for Encke's comet, on this hy- 

 pothesis, was found to be rather longer than the actual, observed period, 

 and, moreover, the duration of the observed period showed a small but 

 regular diminution. Hence, Newton's theory, taken alone, was not 

 sufficient to account for the facts. But, inasmuch as Newton's law of 

 gravitation rested upon a sure and well-established foundation, the fact 

 observed concerning Encke's comet could not be regarded as disproving 

 the law ; hence these facts were to be explained by tracing them to the 

 action of some agent either of known or of, as yet, unknown nature. 



The regularly diminishing period of Encke's comet remained a resid- 

 ual phenomenon, not contradicting the law of gravitation, but awaiting 

 full explanation. 



A residual phenomenon is, then, a phenomenon which is not fully 

 explained by any established theory ; but at the same time it is not a 

 phenomenon which is absolutely contradictory to any such theory, for, 

 if this were the case, the theory in question must perforce be aban- 

 doned. 



Advances are made in natural science by a judicious use of hypoth- 

 eses. Facts are accurately observed, or are gained by exact experiment, 

 and are compared with facts ; inferences are drawn, and are compared 

 with other inferences, until a good working hypothesis is attained. 

 From this hypothesis deductions are made which must necessarily prove 

 true if the hypothesis be correct ; the truth or falsity of the alleged 

 facts is tested by an appeal to Nature ; and so wider hypotheses are 

 gained, each in turn being tested and tried by an appeal to facts, until, 

 finally, that generalization is reached which includes in its expression so 

 many and so varied phenomena that to it is given the name of a " law 

 of Nature." 



But notwithstanding the sure and tried foundations upon which each 

 law of nature rests, phenomena ever and anon become apparent which 

 refuse to be completely explained by any of these laws. Upon more 

 careful examination, it may be found that such phenomena have been 

 erroneously observed, and they may be brought under the application 



