64 THE NATURE OF ANIMAL LIGHT 



surface tlien receives one lumen per square metre. A 

 metre kerze (M.K.) is tlie illumination given by one 

 Hefner candle at one metre distance. 



The brightness of a surface is measured in lamberts or 

 millilamberts. A lambert is "the brightness of a perfectly- 

 diffusing surface radiating or reflecting one lumen per 

 square cm. "A millilambert is 1/1000 lambert. For fur- 

 ther definitions the reader is referred to the reports of the 

 committee on nomenclature of the Illuminating Engi- 

 neering Society. 



Dubois (1886) states that one of the prothoracic organs 

 of PyropJhorus noctilucus has a light intensity of 1/150 

 Phcenix candle of eight to the pound (probably about 

 equivalent to 1/150 candle) and that 37 or 38 beetles (each 

 using all three light organs) would produce light equiva- 

 lent to one Phoenix candle. Langley (1890) found that to 

 the eye the prothoracic organ of Pyrophorus noctilucus 

 gave one-eighth as much light as an equal area of a candle 

 and the actual candle-power of the insect was 1/1600 

 candle. It may be remarked in passing how widely diver- 

 gent these observations are. 



For the flash of the firefly (Photinus pyralis) Coblentz 

 (1912) found variation from 1/50 to 1/400 candle, the 

 predominating values being around 1/400 candle. A coa- 

 tinuous steady glow is sometimes obtained from this insect 

 and it proved to be of the order of 1/50,000 candle. 



Steady sources of light can be more easily measured 

 and we have two records of the light intensity from lumi- 

 nous organisms with continuous light. One of these is a 

 fish, Photoblepharon palpehratus, with a large luminous 

 organ under the eye, of flattened oval shape, 11 X 5 mm., 

 which glows continuously without change of intensity. 



