3-10 ELECTRIC LIGHTING. 



So that any body raised to a temperature above 2,000 will 

 give us all the rays of the sun. Inversely, the spectroscope 

 may thus be enabled to tell us the temperature of the different 

 lights, and it is, perhaps, because some lights do not exceed 

 1,300 that we lose all those rays beyond the yellow. 



" Tyndall has shown that the visible rays of an incandescent 

 wire bear to the invisible rays a much smaller proportion than 

 in the arc, and it is generally assumed that for the same 

 current the arc will give at least z\ times greater light than 

 an incandescent wire. Tyndall's figures are as follows : 



Visible Rays. Invisible Rays. 



Gas i to 24 



Incandescent wire ... i to 23 

 The arc i to 9 



" The requirements of a good electric lamp are first, intense 

 brilliancy ; secondly, great steadiness ; thirdly, duration. 

 The Serrin lamp has the first kind of excellence ; all those 

 lamps based on incandescence excel in the second respect; 

 the Wallace Farmer light is the only one that attains the 

 third point. The Rapieff is perhaps that form which, up to 

 the present, most nearly combines the three requisites, but 

 in reality no lamp has yet been introduced which fulfils all 

 these requirements. 



"The objections to the use of the electric light are : 



i. The deep shadows it throws, 



2. The indifferent carbon that has hitherto been manu- 

 factured for the purpose, which leads to unpleasant sounds, 

 to great variation in the intensity of the light, and to waste. 



3. The difficulty in distributing the light itself. It is so 

 intense, and confined to so small a space, that it does not 

 lend itself to distribution like the gas flame, which occupies 

 a considerable space. 



4. The unsteadiness of the light due to variations in the 

 speed of the engine employed in driving the dynamo machine. 

 There is another cause of variation in the electric arc, and 

 that is the variation in the resistance of the arc itself, for it 



