ON THE NATURE OF LIFE 207 



individualised perceptions. We see the motion of the star through 

 1 -5 seconds of arc, we see light of a particular colour, and hear 

 sound of a particular pitch. Each of these perceptions is there- 

 fore a synthesis of external events effected during the same small 

 fraction of an individual lifetime. Our sensory mechanisms are 

 such that we can make these syntheses, but not some others ; thus 

 there are ether vibrations which are less rapid than those corre- 

 sponding to the red of the spectrum, but we cannot see them, 

 though we can feel them as heat. Below these, again, are others 

 which are much slower, and which we can neither see as light nor 

 feel as heat, though we can detect them by the receiver of a wire- 

 less telegraphic apparatus. Above the violet of the spectrum are 

 vibrations which are so rapid that we cannot see them, though we 

 can make them act upon a photographic plate. Other animals are 

 certainly different from what we are in respect of these matters; 

 thus a dog can certainly see light of lower wave-length than we can, V 

 and some insects can hear sounds which are too acute for our ears. 



The reader will easily see, then, that rhythm of duration varies. 

 Obviously it varies even in ourselves ; thus the familiar experience 

 that time passes more quickly when one is fifty years old than 

 when one is ten means that the later ones have fewer events in 

 them that is, the periods of our duration which correspond 

 with a revolution of the earth round the sun do not gather up so 

 much of other external events when one is old as when one is 

 young. To say that young people have a " fuller " life is not 

 figurative, but is strictly scientific language. 



Now think of how the rhythm of duration varies in different 

 animals. It is probable that the imagines (the fully meta- 

 morphosed insects) of some species of Ephemerida3 which live 

 for only a few hours can appreciate, or feel as distinct lapses of 

 duration, very much smaller intervals of astronomical time than 

 we do. Suppose, for instance, that a period of ^STTOOTJZF second 

 can be felt by them (in which period they would still receive 

 some 8,000 millions of vibrations when looking at red light) ; then 

 their lifetime of a few hours would contain as much (that is, have 

 the same duration) as ours of seventy years. Suppose, again, 

 that some very long-lived animals, such as crocodiles, have 

 unitary periods of duration that are much greater than ours; 

 then their lives would be no longer, for they would contain no 

 more. Note, however, that in such cases the universes con- 

 structed from perception would be different ones. 



