NEW CONCEPTIONS IN SCIENCE 



and that a sunbeam may be broken up into its 

 component colors by means of an ordinary three- 

 cornered prism. A three-cornered glass of water 

 or other liquid does much the same, and that is 

 exactly what the rain -drops do when the sun 

 strikes them right. The colored image produced 

 by the prism is an artificial rainbow. Old Sir 

 Isaac called it a spectrum, and the name has held. 

 It is the same as our word spectre an apparition. 



Curious-minded men were not long in finding out 

 that beyond either end of the visible spectrum 

 curious things go on. For example, if a thermom- 

 eter be held below the red end of this artificial rain- 

 bow, in the infra-red, as it is called, it gets hot, al- 

 though there is not much heat in the visible part of 

 the spectrum. The quite unbearable heat you get 

 with a burning-glass is due to these invisible heat- 

 rays, and not to the light at all. So, again, it is 

 possible to split up a sunbeam with a prism, and 

 then focus only the invisible infra-red part of the 

 spectrum, and get almost as much heat as though 

 you had focussed the entire body of the light. 



Of course if our eyes were sensitive to these in- 

 visible " heat-rays," as they are to the " light-rays," 

 we could "see" with heat just as well as with light. 

 Indeed, we can conceive of a race of men fitted with 

 eyes sensible to the heat-rays, and only to them. 

 To such a race our day would be bright as to us, 



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