Report on tJte Bacteriology of Water. 313 



The spores were sown at 10 A.M. and placed in the light over 

 mirrors at a south window. The temperature was fairly steady at 

 16 C. The light was chiefly reflected from brightly illuminated 

 clouds, but intermittent sunshine touched the mirrors from 11.35 A.M. 

 to 1 P.M., at intervals. 



At 4 P.M. i.e., six hours after sowing, the spores behind the red 

 screens had germinated to rodlets about 10 ft. long, whereas no trace 

 of germination could be observed behind the blue glass. At 8 P.M. 

 the temperature having only fallen half a degree, the average length 

 of the filaments behind the red glass was 125 to 135 /it, but none of 

 the spores behind the blue glass had germinated ont. 



Next morning at 10 A.M. the filaments in the red light were 750 to 

 1,000 n long, as near as I could measure them, but all the spores in 

 the blue were dead, as evinced by their not germinating during the 

 day at 14 18 C., nor during the two further days the culture was 

 kept. 



This experiment, and several others which confirmed it in every 

 respect, shows clearly that in bright sun- light the spores are killed 

 behind blue screens, whereas the same exposure behind red screens 

 leads to no injury whatever confirming the numerous similar results 

 with macroscopic cultures. 



On February 13 a bright, clear blue sky with strong sunshine 

 prevailed all the morning, the afternoon being warm and with bril- 

 liant sunshine, obscured occasionally by clouds. I availed myeelf of 

 the brilliant light reflected from the mirrors, but only allowed direct 

 sunshine (reflected from the mirrors) to play on the objects for a 

 quarter of an hour at the beginning of the exposure. 



The spores had of course all been sown at the same time, so that 

 each culture began under equal conditions. Six sowings were made, 

 each containing about 20 30 spores in the hanging drop of dilute 

 broth. One was kept under a darkened bell-jar, one behind ordinary 

 glass, and the others behind red, blue, green, and pale olive glass 

 respectively.* The exposures began at 11 A.M. at 16 C. ; the tem- 

 perature rose somewhat rapidly to 18 C., and reached 20 21 C. 

 before 2 P.M., and remained at that till after 4 P.M., when it slowly 

 sunk to 16 C. at 8 P.M., and remained at that through the night. 



At 2 P.M. all had begun to germinate, and by 5 P.M. perceptible 

 growth of the filaments could be observed and measured, and so on 

 later. I carried out the measurements as follows, each under the 

 same scale, every division of which (with the power Zeiss D/4 used) 

 corresponded to 5 /*. 



In each drop I sought for the longest and the shortest filament or 



* I owe sincere thanks to Mr. Walker of the Normal School, South Kensington, 

 and to Professor Riicker for much kindness in testing the diathermancy, trans- 

 parency, and other properties of glass screens for me. 



VOL. LVII1. Z 



