WAVE THEORY OF LIGHT 271 



by red light. Violet light, after what we have seen and have 

 had illustrated by that curve (Fie. 119), I need not tell 

 you corresponds to vibrations of about 800 million million 

 per second. There are recognisable qualities of light caused 

 by vibrations of much greater frequency and much less fre- 

 quency than this. You may imagine vibrations having about 

 twice the frequency 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 continuous 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 pho- 

 tographically sensitive 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 fly- 

 ing 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 near, but not over, a set of hot water pipes 

 used for heating a house ; the thing we perceive in our faces 

 and hands when we go near a boiling pot and hold the hand 

 on a level with it, is radiant heat; the heat of the hands and 

 face caused by a hot fire, or by a hot kettle when held under 

 the kettle, is also radiant heat. 



You might readily make the experiment with an earthen 

 teapot; it radiates heat better than polished silver. Hold 

 your hands below the teapot and you perceive a sense of 

 heat; above it you get more heat; either way you perceive 

 heat. If held over the teapot you readily understand that 

 there is a little current of hot air rising; if you put your 

 hand under the teapot you find cold air rising, and the upper 

 side of your hand is heated by radiation while the lower 

 side is fanned and is actually cooled by virtue of the heated 

 kettle above it. 



That perception by the sense of heat, is the perception of 

 something actually continuous with light. We have knowl- 

 edge of rays of radiant heat perceptible down to (in round 



