PHYSICAL NATURE OF ANIMAL LIGHT 65 



The organ can be darkened by a screen similar to an eye- 

 lid which pulls up over it. Steche (1909) reports the inten- 

 sity to be .0024 M.K.* 



Luminous bacteria probably glow with less intensity 

 than any other organism. The light from a single organ- 

 ism cannot be seen but that from a colony is visible to the 

 dark-adapted eye. Even so we must remember that the 

 eye is an exceedingly delicate instrument which can detect 

 very small energy changes. The ^^ minimum radiation 

 visually perceptible ' ' has been calculated by Reeves ( 1917 ) 

 to be in the neighborhood of 18 X 10"^^ ergs per second 

 and the light from a small colony of luminous bacteria 

 represents little more radiation than this. 



Lode (1904, 1908), by a modified grease spot photo- 

 meter method, ascertained that the light of his brightest 

 bacterial colony of Vibrio rumple had an intensity of 

 7.85 X lO-'o H.K. per sq. nmi. or 0.785 H.K. per 1000 

 sq. metres (=0.562 German-normal candles per 1000 sq. 

 metres). In round numbers this is about one German-nor- 

 mal candle per 2000 sq. metres, or two to three times this 

 area for the light from an ordinary stearin candle. Lode 

 calculated that the dome of St. Peter's at Rome, if covered 

 with bacteria, would give little more light than a conmion 

 stearin candle. An ordinary room of 50 sq. metres wall 

 and ceiling area would give out only 0.039 German-normal 

 candle. It does not seem likely that luminous bacteria 

 will ever come into vogue for illuminating purposes. 

 Friedberger and Doepner (1907) by a photographic 

 method, not entirely free from error, found that one 

 square millimetre of lighting surface of a bouillon culture 



* The metre-kerze is a unit of illumination, not of intensity, and is 

 incorrectly used by Steche. 



