1893.] 



The Action of Light on Bacteria. 



473 



tilis, a violet bacillus from the Thames, and several other Thames 

 bacilli being the chief. 



In all cases so far examined, both the solar and electric spectra 

 show that no action whatever is perceptible in the infra-red, red, 

 orange, or yellow region, while all are injured or destroyed in the 

 blue and violet regions. 



The exact point when the action begins and ends is not the same 

 in all the experiments, though very nearly so, but it must be reserved 

 for the detailed memoir to discuss the various cases. 



Broadly speaking, the action begins at the blue end of the green, 

 rises to a maximum as we pass to the violet end of the blue, and 

 diminishes as we proceed in the violet to the ultra-violet regions 

 (fig. 1). 



1. Plate of anthrax spores, exposed for five hours to the solar spectrum in 

 August, and incubated for forty-eight hours. The spectrum shone on 

 the plate through a slot of the width shown by the cleared portion, and 

 whose length is denoted by the base line above the letters. The letters 

 mark the principal regions of the spectrum ; the vertical lines, the limits 

 of these regions (not Fraunhofer's lines). Thus, those radiations we call 

 infra-red, red (K), orange, and yellow affected the spores no more than 

 total darkness, and colonies, therefore, germinated out in those regions as 

 readily as over the main area of the plate. The same is partly true of the 

 green (G-) and the violet and ultra-violet regions to the right of V. The 

 maximum effect is in the blue and blue-violet (BV), where nearly every 

 spore has been destroyed, and the area appears cleared of colonies. 



