18 THE NATURE OF ANIMAL LIGHT 



in fishing, of the luminous organ of a fish, PJiotohlepharon, 

 by the Banda islanders, we find that luminous bacteria are 

 of value for certain purposes in the laboratory. 



These methods are all due to Beijerinck (1889, 1902). 

 He has, for instance, used luminous bacteria for testing 

 bacterial filters. If there is a crack in the filter the bac- 

 teria will pass through and a luminous filtrate is the 

 result, but a perfect filter allows no organisms to pass 

 and gives a dark filtrate. 



Luminous bacteria are also very sensitive to oxygen 

 and cease to luminesce in its absence. By mixing luminous 

 bacteria with an emulsion of chloroplasts (from clover 

 leaves) in the dark, allowing the bacteria to use up all the 

 oxygen, and then exposing the mixture to light of various 

 colors, the effect of different wave-lengths in causing 

 photos^aithesis could be studied. Only if the chloroplasts 

 are exposed to a color in the spectrum which decomposes 

 CO2 with liberation of oxygen do the bacteria luminesce, 

 and when this oxygen is used up by the bacteria, the tube 

 again becomes dark. Beijerinck has also worked out a 

 method of testing for maltose and diastase with luminous 

 bacteria, based on the fact that a certain form, Photohac- 

 termm pJiosphorescens y will only produce light in pres- 

 ence of maltose or diastase which will form maltose 

 from starch. 



Although Dubois and Molisch have both prepared 

 ^ ' bacterial lamps ' ^ and although it has been suggested that 

 this method of illumination might be of value in powder 

 magazines where any sort of flame is too dangerous, it 

 seems doubtful, to say the least, whether luminous bac- 

 teria can ever be used for illumination. Other forms, 

 perhaps, might be utilized, but bacteria produce too weak 



