CHAPTER X. 

 PHOTOMETRY AND ELECTRIC LIGHTING. 



125. Radiant heat. Light. The radiation from a hot body 

 may be resolved into simple parts each of which is a train of ether 

 waves of definite wave-length. All of these simple parts of the 

 total radiation have one common property, namely, they generate 

 heat in a body which absorbs them. Therefore every portion of 

 the radiation from a hot body is properly called radiant heat. 

 The intensity of a beam of radiant heat is measured by the heat 

 it delivers per second to an absorbing body. Thus the radiation 

 emitted by a standard candle represents a flow of about 450 ergs * 

 per second across one square centimeter of area at a distance of 

 one meter from the candle. 



Radiant heat of which the wave-length lies between 39 and 75 

 millionths of a centimeter affects the optic nerves and gives rise 

 to sensations of light. Therefore radiant heat of which the wave- 

 length lies between these limits is called light. These limits, 

 which are called the limits of the visible spectrum, are not sharply 

 defined, but they vary considerably with different persons and 

 with the degree of fatigue of the optic nerves. 



The physical intensity of a beam of light is measured by its per- 

 fectly definite thermal effect, that is by the heat energy it delivers 

 per second to an absorbing body. Thus those parts of the radi- 

 ation of a standard candle which lie within the visible spectrum 

 represent a flow of about 9.3 ergs per second across an area of 

 one square centimeter at a distance of one meter from the candle. 

 Comparing this with the flow of energy which is represented by 

 the total radiation from a standard candle, namely, 450 ergs per 



* Heat is properly expressed in energy units. One erg of heat is the amount of 

 heat that is equivalent to one erg of mechanical energy. 



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