ARTIFICIAL LIGHTS. 13 



largely employed. When burned under the most favor- 

 able conditions, kerosene will yield a light equal to 

 thirty or forty candles, and will illuminate a disk eight 

 or ten feet in diameter very well. 



Illuminating gas and kerosene have this advantage, 

 that they are very cheap, costing but a few cents an 

 hour ; but they can only be used to enlarge pictures 

 which are very transparent, outline drawings, or chem- 

 ical reactions in the large tank, which will be described 

 further on. 



There is no absolute standard of the luminous inten- 

 sity of light. A conventional standard of its intensity 

 is used for convenience, and for regulating the illu- 

 minating power of gas. In Massachusetts, the legal 

 standard is the light of a sperm candle weighing two 

 and two-thirds ounces, when consuming one hundred 

 and twenty grains in an hour. In some places wax 

 candles are adopted, but the light does not differ much 

 in intensity from that given by sperm candles. 



With such a standard as this, the various lights 

 which have been specified, have the following relative 

 illuminating powers under favorable circumstances : 



Illuminating gas, 15 



Coal oil in Argand burner, . . . 20 



Coal oil in Marcy's lamp, . . . .25 



Magnesium, 40 



The lime light with oxygen and alcohol, . . 50 



" " " " " " common gas, TOO 



" " " hydrogen, . 125 



The Electric light, . . . 500 to 10,000 



All of these are very variable, especially the lime 

 light, which depends upon the quality and pressure of 

 the gases, the quality of the stick of lime, and the kind 

 of jet used. 



The electric light has a very wide range in intensity, 



