ACTION OF LIGHT ON MICRO-OEGANISMS 353 



An exposure for three, four, or six hours to the sun 

 in September and October was found also capable of 

 modifying the pigment-producing powers of this bacillus, 

 although the colour was not so uniformly removed, and 

 a tendency to return to its original colour was notice- 

 able. 



Laurent explains the presence of pink colonies 

 amongst the colourless centres as due to inequalities in 

 the power of resistance possessed by individual bacilli, 

 as well as to the uneven thickness of the growth in 

 difierent parts of the potato-slice. 



Some investigations were also made to determine 

 whether the colour would be similarly affected in 

 cultures kept in an atmosphere of hydrogen or carbonic 

 acid, and it was found that it was only in the presence 

 of air that insolation produced its full effect, and that a 

 permanently colourless race of bacilli was obtained. 



Laurent also found that all the rays of the spectrum 

 acted prejudicially on the production of the pigment, 

 but that the principal part in this respect was played 

 by the most highly refrangible rays of the spectrum. 



Still more recently, d'Arsonval and Charrin ^ have 

 examined the action of sunshine on the pigment-pro- 

 ducing power of the B. pyocyaneus. These investigators 

 state that when this organism is exposed in a liquid 

 (the nature of the medium is not mentioned, but it was 

 presumably broth) for from 3 to 6 hours to sunshine, 

 and is afterwards introduced (one drop being used) into 

 agar-agar, and kept at 35° C, only colourless colonies 

 make their appearance ; if, however, it is exposed to 

 the red rays only, the typical fluorescent green-coloured 

 colonies are produced. If the exposure to sunshine is 



^ ' Influence des Agents atmosph^riques, en particulier de la Lumifere, 

 du Froid, sur le Bacille pyocyanogfene.' CoTriptes rendus, vol. cxviii. 

 1894, p. 151. 



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