INFLUENCE OF LIGHT AND OTHER RAYS 



279 



the amount of moisture and dust in the atmosphere, with the time of 

 the day and with the season; an absolute measure for the action of light 

 cannot be fixed, therefore, as easily as with the action of heat in the ther- 

 mal death-point. The different colors of the spectrum do not act 

 alike; the part of the spectrum from red to green is practically without 

 influence upon microorganisms, while the blue light acts strongest 

 and the intensity decreases in the violet and ultra-violet. In carrying 

 on experiments with the influence of light, it must be remembered that 

 glass absorbs ultra-violet rays, and further that the heating of the 

 medium by direct radiation must be avoided (Fig. 117). 



FIG. 1 1 8. Phototropism of Rhizopus nigricans. The mold is grown on gelatin with 

 diffused light coming from right side. (Original.) 



Yeasts, molds, and bacteria and probably Protozoa are equally sensi- 

 tive to light. Even the spores of most bacteria do not show a greater 

 resistance to light, while the mold spores are an exception. The col- 

 ored spores of the Penicillium, Aspergillus and Mucor species can be 

 exposed to light for a long time without being killed, but the colorless 

 spores of Oidium and Chalara show no increased resistance. It is sup- 

 posed that the pigment in mold spores is a protection against light. This 

 is not true with the pigment of bacteria. The colored and colorless 

 strains of pigmented bacteria show no difference in their resistance to 

 light. The only exceptions are the so-called purple bacteria. These 

 peculiar organisms, many of which feed on hydrogen sulphide, seem to 



