XVIII.] OBSERVATION. 405 



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 lidit 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 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 because they are never heard. 

 Insects may communicate by such acute sounds, con- 

 stituting a language inaudible to us ; and the remarkable 

 agreement apparent among bodies of ants or bees might 

 thus perhaps be explained. Nay, as Fontenelle long ago 

 suggested in his scientific romance, there may exist un^ 

 limited numbers of senses or modes of perception which 

 we can never feel, though Darwin's theory would render it 

 probable that any useful means of knowledge in an an- 

 cestor 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. 

 It is now apparent that what we call light is the affection 

 of the eye by certain vibrations, the less rapid of which 

 are invisible and constitute the dark rays of radiant heat, 

 in detecting which we must substitute the thermometer 

 or the thermopile for the eye. At the other end of the 

 spectrum, again, tlie ultra-violet rays are invisible, and 

 only indirectly brought to our knowledge in the pheno- 

 mena of fluorescence or photo-chemical action. There is 

 no reason to believe that at either end of the spectrum an 

 absolute limit has yet been reached. 



Just as our knowledge of the stellar universe is limited 



