178 TABLES 166-168. 



TABLE 168. -Photometric Standards. 



No primary photometric standard has been generally adopted by the various governments. In 

 Germany the Herner lamp is most used ; in England the Pentane lamp and sperm candles are 

 used ; in France the Carcel lamp is preferred; in America the Pentane and Hefner lamps are used 

 to some extent, but candles are more largely employed in gas photometry. For the photometry 

 of electric lamps, and generally in accurate photometric work, electric lamps, standardized at a 

 national standardizing institution, are commonly employed. 



The " International candle " is the name recently employed to designate the value of the candle 

 as maintained by cooperative effort between the national laboratories of England, France, and 

 America; and the value of various photometric units in terms of this international candle is given 

 in the following table (taken from Circular No. 15 of the Bureau of Standards). 



i International Candle = i Pentane Candle. 

 i International Candle = i Bougie Decimale. 

 i International Candle = i American Candle. 

 i International Candle = 1.11 Hefner Unit. 

 I International Candle = 0.104 Carcel Unit. 



Therefore i Hefner Unit = 0.90 International Candle. 



The values of the flame standards most commonly used are as follows : 



1. Standard Pentane Lamp, burning pentane 10.0 candles. 



2. Standard Hefner Lamp, burning amyl acetate 0.9 candles. 



3. Standard Carcel Lamp, burning colza oil 9.6 candles. 



4. Standard English Sperm Candle, approximately .... i.o candles. 



Slight differences in candle power are found in different lamps, even when made as accurately 

 as possible to the same specifications. Hence these so-called primary standards should be them- 

 selves standardized. 



TABLE 167. Intrinsic Brightness of Varions Light Sources. 



Taken from Data, 1911. 

 TABLE 168. Visibility ol White Lights. 



1 Paterson and Dudding. * Deutsche Seewarte. o 



The energy falling on i sq. cm. at im. from a candle is about 4 ergs per sec. (Rayleigh, about 8 according to Ang- 

 strom.) 



