1893.] 



Action of Light on Bacillus anthracis. 



29 



On January 16, the plate behind the thick blue screen showed an 

 extremely faint letter; that behind the thinner and much more trans- 

 parent orange screen, a letter, perhaps as distinct, but also extremely 

 faint. The inhibition here was very slight indeed, and the effects 

 were obliterated after a few hours' further incubation ; moreover, I 

 found that with a stronger solution of the potassium bichromate than 

 1 could use in the very cold weather, the inhibiting rays are kept 

 out altogether. 



On January 14 an agar plate of anthrax spores, made the previous 

 day and kept for twenty hours at 5 — 7° C, w T as exposed for five hours 

 to brilliant sunlight behind a thick screen of ammoniated cupric oxide ; 

 the light had to traverse the two glass faces of the screen (each 

 3 3 2- inch), and that of the plate — the bottom of a Petri's dish — as well 

 as the solution, which was ^ inch thick. When held up to the sun, 

 the latter was just plainly visible through the deep violet blue solu- 

 tion ; but the letter T of the stencil plate covering the culture was 

 perfectly invisible. 



On the same date, and exposed for the same time, side by side 

 with the above culture, an exactly similar plate poured from the same 

 tube, &c, was put behind a screen of 'potassium bichromate, which was 

 so transparent that the letter (also a T) of the stencil plate was 

 distinctly visible through it. Moreover, this screen was only inch 

 thick, and each glass face y 1 ^ inch. 



On January 16, i.e., after little more than twenty-four hours' in- 

 cubation, the plate from behind the blue screen, showed a perfectly 

 sharp letter T, whereas there was no trace as yet on that from behind 

 the orange screen ; the latter plate, however, showed an extremely 

 faint letter after two more days' incubation, proving that the rays 

 transmitted through the potassium bichromate solution were slightly 

 inhibitory, though very much less so than those passing the am- 

 moniated cupric oxide. 



Further experiments, in which the two screens were interchanged, 

 only confirmed these results, and carried them a step further, showing 

 that the light passing through the blue solution, whether in the thick 

 or the thin screen, always gives a sharp and clear letter, whereas that 

 through the orange solution, exposed side by side, and for the same 

 time, is always very faint, if it comes out at all, and it only appeared 

 in my earlier experiments, when, owing to the low temperatures, the 

 bichromate solution was too dilute. Not only so, but the very dim 

 blue light (as estimated by the eye) which traverses the thick screen 

 of copper salt is far more bactericidal than the much brighter orange 

 light transmitted through the thinner screen. In other words, the 

 more refrangible blue rays are the effective ones in destroying the life 

 of the spores. 



