288 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



ultravioJet rays is very small compared with that of the red and infra- 

 red rays. The heat emitted by a candle at a distance of 1 meter would 

 raise the temperature of 1 gram of water, hardly a thimbleful, by only 

 1° 0. in one and one-fourth years, provided it could be stored for that 

 length of time. This will give you some notiou of the extreme sensi- 

 tiveness of the eye for light rays. But nevertheless it takes a definite 

 quantity of energy, a very small quantity, to produce the sensation of 

 light. Hence a body radiating light waves of this intensity j ust begins 

 to become visible. This incipient incandescent state begins at a rela- 

 tively high temperature in the sources of light commonly used. It is 

 not that these waves are not present at lower temperatures, but their 

 energy is too small to affect the optic nerve. Just as soon as the crit- 

 ical temperature is passed, the heated body becomes luminous and its 

 brightness increases rapidly with increasing temperature. The amount 

 of energy producing the sensation of light is however a minute fraction 

 of the total energy radiated by the body. This total radiation obeys 

 definite laws iu respect to its increase with the temperature, and so 

 does that x)ortion of it which affects the eye. Before considering these 

 laws, let us first see how these two kinds of radiation can be separated. 

 Separation of heat radiation from light radiation. — I have here a strip 

 of sheet platinum tlirough which an electric current of any strength 

 up to 100 amperes can be transmitted. [Experiment.] 

 The current heats the strip, and as it increases the 

 temperature of the metal increases until it finally begins 

 to glow, and by a still further increase it is heated to 

 intense whiteness and then melts. Before it begins to 

 glow it nevertheless emits energy, for the heat radiated 

 by it can readily be felt by placing the hand near it. In 

 order to show this development of heat to all of you 

 I make use of a so- called " differential thermometer" 

 Fig. 3. ^flg._ 3^^ j^ consists of two glass bulbs connected by a 



tube acb, which contains a quantity of liquid separating the air iu the 

 two bulbs. This liquid column is displaced when either bulb is heated. 

 On the projection screen you see an enlarged image of one of the liquid 

 surfaces. As soon as I bring up the platinum strip, through which the 

 current is passing, to one of the bulbs previously coated with lampblack, 

 the level of the liquid immediately begins to change, although the plati- 

 num is not emitting visible radiations. The ether waves emitted are 

 simply absorbed by the lampblack and transformed into heat, which is 

 imparted to the inclosed air. This expands, and the liquid is forced 

 over into the other bulb. 



The higher the temperature of the platinum the more rapidly does 

 the air of the diii'erential thermometer expand when the platinum is 

 brought near it. Now it glows with dull redness, and the level of 

 the liquid is displaced beyond the limits of the screen. But as soon as 

 a glass plate, or still better a vessel containing water, is interposed 



