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as light without attempting to define conscious- 

 ness, because we cannot define consciousness any 

 more than we can define free will we shall be 

 safe. There is no question that you see the 

 thing : if you see it, it is light. Well now, when 

 is radiant heat light ? Radiant heat is light 

 when its frequency of vibration is between 400 

 million millions per second and 800 million 

 millions per second. When its frequency is less 

 than 400 million millions per second it is not 

 light ; it is invisible " infra-red " radiant heat. When 

 its frequency is more than 800 million millions per 

 second it is not light since we cannot see it ; 

 it is invisible ultra-violet radiation, truly radiant 

 heat, but it is not so commonly called radiant 

 heat because its heating effect is known rather 

 theoretically than by sensory perception, or 

 thermometric or thermoscopic indications. Ob- 

 servations which have been actually made by 

 Langley and by Abney on radiant heat take us 

 down about three octaves below violet, and we 

 may hope to be brought considerably lower still 

 by future observation. We know at present in all 





