16 THE PRINCIPLES OF SCIENCE. 



luminous by auroral currents of electricity, or by the 

 passage of meteoric stones. 



There are many phenomena in meteorology and other 

 similar sciences, in which some occurrences depend on 

 others for their visibility. Thus in estimating the com- 

 parative numbers of meteors seen in different months of 

 the year, it is essential to take account of the varying 

 frequency of cloudy weather or else of the different 

 duration of the daylight which hides all but the most 

 splendid meteors. Observations of the comparative fre- 

 quency of various kinds of clouds will be complicated by 

 the fact that dense rain clouds necessarily hide those more 

 delicate cirrous clouds which appear in the higher parts 

 of the atmosphere. Most of the visible phenomena of 

 comets probably arise from some substance which, existing 

 previously invisible, becomes condensed or electrified sud- 

 denly into a visible form. Sir John Herschel attempted 

 to explain the production of comet tails in this manner by 

 evaporation and condensation 1 ". 



Negative Arguments founded on the Non- observation of 



Phenomena. 



From what has been suggested in preceding sections, it 

 will plainly appear that the non-observation of a pheno- 

 menon is not generally to be taken as proving its iion- 

 occurrence. As there are sounds which we cannot hear, 

 rays of light which we cannot feel, indefinite multitudes 

 of worlds which we cannot see, and infinite myriads of 

 minute organisms of which not the most powerful micro- 

 scope can give us a view, we must as a general rule 

 interpret our experience in an affirmative sense only. 

 Accordingly when inferences have been drawn from the 

 non-occurrence of particular facts or objects, more ex- 



r ' Astronomy,' 4th ed. p. 358. 



