272 THE PRINCIPLES OF SCIENCE. 



atmosphere to prevent the existence of undulations in- 

 comparably more rapid than aDy of which we are con- 

 scious, we may infer, by the principle of continuity, that 

 such undulations probably exist. 



There are many habitual actions which we perform we 

 know not how. So rapidly are many . acts of mind ac- 

 complished that analysis seems impossible. We can only 

 investigate them when in process of formation, observing 

 that the best formed habit or instinct is slowly and con- 

 tinuously acquired, and it is in the early stages that we 

 can perceive the rationale of the process. 



Let it be observed that this principle of continuity 

 must be held of much weight only in exact physical 

 laws, those which doubtless repose ultimately upon the 

 simple laws of motion. If we fearlessly apply the prin- 

 ciple to all kinds of phenomena, we may often be right in 

 our inference, but also often wrong. Thus, before the 

 development of spectrum analysis, astronomers had ob- 

 served that the more they increased the powers of their 

 telescopes the more nebulae they could resolve into dis- 

 tinct stars. This result had been so often found true 

 that they almost irresistibly assumed that all the nebuloe 

 would be ultimately resolved by telescopes of sufficient 

 power ; yet Mr. Huggins has in recent years proved by 

 the spectroscope, that certain nebulae are actually gaseous, 

 and in a truly nebulous state. Even one such observation 

 is a real exception sufficient to invalidate previous in- 

 ferences as to the constitution of the universe. 



The principle of continuity must have been continually 

 employed in the inquiries of Galileo, Newton, and other 

 experimental philosophers, but it appears to have been 

 distinctly formulated for the first time by Leibnitz. He 

 at least claims to have first spoken ' of the law of con- 

 tinuity ' in a letter to Bayle, printed in the ' Nouvelles de 

 la Republique des Lettres/ an extract from which is 



