GENERAL ACCOUNT OF RADIATION. 



223 



Then again, radiant energy and light have the same laws of propaga- 

 tion. The amount radiated varies with the slope of surface, just as in 

 the case of light, and the quantity of radiant energy falling on a given 

 area, from a given source in a given time, varies inversely as the square 

 of the distance the law of light-propagation. 



This may be proved very simply by placing a thermopile in front of 

 a blackened tin vessel full of hot water, and between the two a screen 

 with a hole in it (Fig. 131). So long as the hot vessel entirely fills up the 

 field of view of the thermopile through the hole in the screen, the 

 deflection of the galvanometer remains the same, whatever the distance 

 between the screen and the hot vessel, provided that the distance of the 

 thermopile from the hole in the screen remains unchanged. That this 

 proves the law may be seen from a simple case. Let the hot vessel be 

 placed successively at two distances 1 and 2 respectively from the pile, 

 the screen remaining fixed. Then the areas seen from the pile, and 

 from which it is receiving radiation, are as 1:4. But the total energy 

 received is the same in each 

 case. Then the energy received 

 from the single area at the 

 double distance is J of that 

 received from it at the single 

 distance. Further, we may alter 

 the slope of the hot vessel 

 without affecting the heat re- 

 ceived by the thermopile, just 

 as we may alter the slope of a 

 light-giving surface. Common 

 observations and simple experi- 

 ments, therefore, show us that 

 radiant energy resembles light 

 in its most evident properties. 

 We may supplement these 



observations and experiments by various other experiments, and they 

 all confirm the conclusion. 



Light is Radiant Energy to which the Eye is Sensitive. 



Formerly, it was thought that this resemblance was all that we could 

 assert and that corresponding to our two distinct sense-perceptions, one 

 by the retina and the other by the warmth-sense in the skin, there were 

 two distinct external agents travelling together. This was quite natural 

 before physicists had begun to think of phenomena in terms of energy 

 and its transformations. But now we clearly realise that light itself is 

 a form of energy sent out by a heated source, which, in parting with the 

 light, loses an equivalent of heat, and that this light energy, falling on 

 an absorbing surface, is transformed into heat again. In this respect 

 it is undistinguishable from radiant energy. We have now no 

 reason whatever to suppose that there is the double agency. When the 

 radiant energy falls on the retina if it is of suitable quality it gives 

 rise to the sensation of light. The same energy falling on an absorbing 

 substance, say, a blackened thermometer, will be transformed into heat. 

 The difference, then, between light as received by the eye and radiant 

 energy as studied by the thermometer, thermopile, or other radiometer is 



FIG. 131. Proof of the Law of 

 Inverse Squares. 



