April io, 1902 J 



NA TURE 



537 



the agitation only resulting in the introduction of fresh 

 oxygen and not as being a direct exciting cause. 



The number of species isolated up to the present is 

 about twenty-five, but it is more than probable that some 

 of these are identical, or at any rate closely related. 



In artificial cultivations, these organisms grow best on 

 a medium containing a considerable percentage of a 

 soluble chloride in addition to the nutritive material. 

 They will grow on an ordinary peptone-beef-broth gelatine 

 medium, but they do not all emit light, and none of them 

 emit the maximum amount they are capable of producing. 

 The best results are to be obtained by adding to the 

 culture medium 2'6 per cent, of sodic chloride, 075 per 

 cent, of magnesia chloride, and '3 per cent, of potassic 

 chloride. 



Either of the chlorides which occur in sea-water, if 

 added to a nutritive medium in suitable proportions, will 

 cause some luminosity, but the results are not so good as 

 on the medium mentioned. 



In the case of fluid nutrient media, some means must 

 be taken to replenish the oxygen, as the amount held in 

 solution is speedily exhausted. Either free oxygen can 



Fl<;. 3. Plats culture of luminous bacteria. 



be allowed to bubble through the medium, in which case 

 very brilliant cultures can be obtained, or frequent agita- 

 tion can be resorted to. 



The temperature at which these organisms grow is 

 variable. Those found in northern latitudes can grow 

 and remain luminous at o' C, the optimum temperature 

 being about 15^ at which reproduction is very rapid 

 and luminosity at its maximum. .Some organisms found 

 m the tropics grow, however, at a much higher tempera- 

 ture, but none of them have an optimum as high as 

 blood-heat, 37' C. Spectroscopically, the light emitted 

 by these organisms is confined to a small portion of the 

 visible spectrum, never extending into the ultra-violet or 

 infra-red. \"isually it only includes the green and blue, 

 and photographically it extends very slightly further 

 towards the violet. 



Fig. 1 shows a photograph of the spectrum of this 

 organism (ri), with the spectrum of hydrogen beneath for 

 reference (/;). It will be seen that the former is continuous, 

 and the brightest portion lies between the lines v and G. 

 There is some extension towards the u line, but it is not 

 well marked, and it is only with very long exposure that 



NO. 1693, VOL. 65] 



this becomes evident. This result has been confirmed 

 by using a spectroscope with a quartz system, but owing^ 

 to its low dispersion, the photograph here shown was 

 regarded as more suitable for purposes of illustration. 



Figs. 2, 3, 4 and 5 are all cultures of luminous organ- 

 isms, photographed entirely by their own light. . 



Fig. 2 is a photograph of three growths on gelatine to 

 show the power of emitting light for long periods. The 

 first one («) is the oldest growth, some seven weeks old, 

 in which, as always happens, the light has diminished at 

 the centre of the streak, but is still bright at the edges, 

 where reproduction of the organisms is still taking place. 

 The next 'b) is a three weeks' growth, while \c) is a young 

 culture, showing that the streak is equally bright through- 



out. Fig. 3 is a plate cultivation one week old to show the 

 individual colonies. Fig. 4 is a single colony magnified 

 about forty-five diameters, showing that even in young 

 bright growths the emission of light is greatest at the edges. 



