8o THE BACTERIA UNDER PHYSICAL AGENCIES. 



first member of which resists the action of diffused daylight the 

 longest : Bacillus ruber, Micrococcus prodigiosus, Sarcina rosea, 

 Bacillus vwlaceus, B. pyocyaneus, B. lad is erythrogenes. Accord- 

 ing to the researches of GROTENFELT (I.), the last-named fission 

 fungus does not produce red colouring matter at all when strongly 

 illuminated. R. DUBOIS (I.) ascertained that the luminous 



FIG. 29. Thickly-sown plate culture of typhus bacilli on agar-agar. Covered with paper 

 letters and exposed to the sun's rays for i hours, then kept twenty-four hours in the 

 dark, whereupon development of thickly congregated whitish colonies was found 

 only at the parts covered by the letters. (After H. Buchner.) Nat. size. 



bacterium, PJiotobacterium sarcopkilum, found on spontaneously 

 phosphorescent flesh, temporarily loses its light-producing power 

 on prolonged exposure in a light room. 



Great differences in susceptibility to sunshine are also ex- 

 hibited in the Scliizomycetes. At the extreme end of the series 

 stand the purple bacteria, examined more closely by Engelmann, 

 which always seek out the more highly illuminated positions. 



