172 



LIGHT AND PROTOPLASM 



[Cn. VII 



light for 6 hours, and if then the whole plate is placed in a dark 

 incubator at 20 C. for 48 hours, the bacteria will be found to 

 have developed in all parts of the plate except in the ^-shaped 

 area sterilized by the light (Fig. 47). Compare the earlier 

 results of BUCHXER ('92). That in these cases it is the light 

 and not a high temperature which induces the sterilization in 

 the illuminated region is shown by the fact that BUCHNER ('92) 

 obtained even more striking results when parts of the culture 

 plate were exposed under 50 cm. of water, which cuts off the heat. 



FIG. 47. Appearance of a gelatine culture of Bacillus anthracis, exposed to the light 

 over only the area E, and then incubated for 48 hours. In the area E no colonies 

 have developed. (From WARD, '93.) 



Not all rays have this bactericidal property. DOWNES and 

 BLUNT ('78) found that only the blue rays were thus active, 

 for behind red or yellow glass the bacteria readily developed. 

 WARD ('94) threw a solar spectrum upon an agar film in which 

 bacteria were developing in a dark chamber. He found that 

 the bactericidal effect was greatest in the region of the blue- 

 violet rays (about X = 0.43 /A) and diminished towards the 

 extreme violet arid the yellow, where it had almost disappeared. 

 These facts were ascertained by incubating the bacteria for 48 

 hours after insolation, when certain parts affected by the spec- 

 trum were found to remain clear (Fig. 48). When an electric 



