THE PRINCIPLES OF SCIENCE. 



effects upon luminous stars. Thus if one member of a 

 double star were dark, we could readily detect its exist- 

 ence, and even estimate its size, position, and motions, 

 by observing those of its visible companion. It was a 

 favourite notion of Huyghens that there may exist stars 

 and vast universes so distant that their light has never 

 yet had time to reach our eyes ; and we must also bear 

 in mind that light may possibly suffer slow extinction 

 in space, so that there is more than one way in which 

 an absolute limit to the powers of telescopic discovery 

 may exist. 



There are natural limits again to the power of our 

 senses in detecting undulations of various kinds. It is 

 commonly said that vibrations of less than sixteen strokes 

 or more than 38,000 strokes per second are not audible as 

 sound ; and as some ears actually do hear sounds of much 

 higher pitch, even two octaves higher than what other 

 ears can detect, it is exceedingly probable that there 

 are incessant vibrations which we cannot call sound be- 

 cause they are never heard. Insects may possibly com- 

 municate by such acute sounds, constituting a language 

 inaudible and inscrutable to us ; and the remarkable agree- 

 ment apparent among bodies of ants or bees might thus 

 perhaps be explained. Nay, as Fontenelle long ago sug- 

 gested in his scientific romance, there may exist unlimited 

 numbers of senses or modes of perception which we can 

 never feel, though Darwin's theory would render it pro- 

 bable that any useful means of knowledge in an ancestor 

 would be developed and improved in the descendants. 

 We might doubtless have been endowed with a sense 

 capable of feeling electric phenomena with acuteness, so 

 that the positive or negative state of charge of a body 

 could be at once estimated. The absence of such a sense 

 is probably due to its comparative uselessness. 



Heat undulations are subject to the same considerations. 



