INFLUENCE OF LIGHT AND BONTGEN BAYS. 47 



tion on fresh medium in the dark. With still longer action 

 of the direct sunlight the micro-organisms die. 



Bact. putidum and Bact. prodigiosum were materially disturbed in 

 their ability to produce pigment and trimethylamin by direct sun- 

 light, in July and August in one-half hour and in November in one 

 and a half hours. They grew slowly and prodigiosum liquefied slowly 

 ( Dieudonne^). Death was produced in these organisms in one and a 

 half and two and a half hours. 



Dieudonne (A. G. A. ix, 405 and 537, also an extensive 

 Review of literature) has found ultra-violet, violet, and blue 

 light to be very injurious, green but slightly, and red and 

 yellow not at all. On the contrary, Beck and Schultz 

 (Z. H. xxiii, 490), who employed better light-filters, gen- 

 erally observed no injury from the colored light obtained 

 from sunlight. This is to be explained by the slight in- 

 tensity of the light employed. Also, Beck and Schultz 

 deny that diffuse daylight works injury to bacteria (the 

 chromogenic function is lost in many varieties even when 

 grown in the dark), while Dieudonne asserts : 



In diffuse daylight, in spring and summer in three and a half hours 

 and in winter in four and a half hours, there occurs interference with 

 growth, while in from five to six hours death is produced. Electric 

 arc light of 900 candle-power checks growth in five hours and kills in 

 eight hours. Incandescent light injures growth in from seven to eight 

 hours, and kills in eleven hours. Similar results occur with Bact. 

 coli, typhi, and B. anthracis. Naturally, Dieudonne's positive results 

 are not disproved by Beck and Schultz 's negative ones. 



For testing the sensitiveness to light, thickly sown 

 gelatin or agar plates are exposed to diffuse or direct sun- 

 light, after the method of H. Buchner, a dark paper cross 

 being placed upon the illuminated side. To exclude the 

 action of heat, * one may first carry the light through a 

 layer of water or alum a few centimeters in thickness. 

 After the light has acted for one-half, one, one and a half, 

 two, etc., hours, the plates are placed in a dark room, and 

 it is observed whether the bacteria develop only on the 

 part covered by the cross ; in complete death of the ex- 

 posed bacteria there occurs a sharply outlined cross con- 

 sisting of the colonies in a clear field. 



1 The action of the heat is entirely without interest. 



