212 THE PRINCIPLES OF SCIENCE. 



Residual Phenomena. 



Even when all the experimental data employed in the 

 verification of a theory are sufficiently accurate, and the 

 theory itself is sound, there may still exist discrepancies 

 demanding further investigation. Sir John Herschel was 

 perhaps the first who pointed out the importance of such 

 outstanding quantities, and called them residual pheno- 

 mena). Now if the observations and the theory be really 

 correct, such discrepancies must be due to the incomplete- 

 ness of our knowledge of the causes in action, and the 

 ultimate explanation must consist in showing that there 

 is in action 



(1) Some agent of known nature whose presence was 

 not suspected. 



(2) Some new agent of unknown nature. 



In the first case we cannot be said to make any new 

 discovery, for our ultimate success consists merely in 

 reconciling the theory with known facts when our in- 

 vestigation is more comprehensive. But in the second 

 case we meet with a totally new fact, which may 

 lead us to whole realms of new discovery. Take the 

 instance adduced by Sir John Herschel. The theory of 

 Newton and Halley concerning cometary motions was 

 that they were gravitating bodies revolving round the 

 sun in oblique orbits, and the actual return of Halley's 

 Comet, in 1758, sufficiently verified this theory. But, 

 when accurate observations pf Encke's Comet came to be 

 made, the verification was not found to be complete. 

 Each time Encke's Comet returned a little sooner than 

 it ought, the period having regularly decreased from 

 121279 days, between 1786 and 1789, to i2io - 44 be- 



J ' Preliminary Discourse on the study of Natural Philosophy,' 1 58, 

 174. 'Outlines of Astronomy/ 4th. edit. 856. 



