316 POPULAR LECTURES AND ADDRESSES. 



second. There are recognisable qualities of light 

 caused by vibrations of much greater frequency 

 and much less frequency than this. You may 

 imagine vibrations having about twice the fre- 

 quency of violet light, and others having about 

 one-fifteenth the frequency of red light and still 

 you do not pass the limit of the range of con- 

 tinuous phenomena only a part of which constitutes 

 visible light. 



When we go below visible red light what have 

 we ? We have something we do not see with 

 the eye, something that the ordinary photographer 

 does not bring out on his photographically sen- 

 sitive plates. It is light, but we do not see it. It 

 is something so closely continuous with visible 

 light, that we may define it by the name of 

 invisible light. It is commonly called radiant 

 heat ; invisible radiant heat. Perhaps, in this 

 thorny path of logic, with hard words flying in 

 our faces, the least troublesome way of speaking 

 of it is to call it radiant heat. The heat effect 

 you experience when you go near a bright hot 

 coal fire, or a hot steam boiler ; or when you go 



