1893.] 



Action of Light on Bacillus anthraois. 



25 



Table II (p. 26) summarises the results of a series of experi- 

 ments made with these glass screens, and shows at a glance all the 

 essential points brought out in the experiments. 



The first column merely gives the number of the experiment. 

 Column 2 is occupied with the tabular number of the particular glass 

 screen employed, and is more fully elucidated by reference to the 

 properties summarised in Table I. In column 3 we have the kind of 

 plate employed. In all cases the ripe spores were used, and it only 

 remains to add that it makes no essential difference to the result 

 whether agar, or gelatine, or a mixture of both, supplies the food 

 material, except that agar is, of course, much the most convenient 

 medium for incubation. With regard to Experiments 11 and 12 in 

 this table, the backs of the lids of the Petri's dishes were slightly 

 blackened in the smoke of a candle, in order to reduce the internal 

 reflection, and I ought to state that numerous trials have shown that 

 this does not seriously affect the results. 



It has the disadvantage, however, of sometimes causing black 

 spots on the plates, owing to the condensed water drops carrying off 

 some of the soot ; of course this spoils the neatness of the cultures, 

 and, I confess, I was surprised that nothing worse happened in spite 

 of the slight acidity of these drops. 



Columns 4 and 5 simply give the dates of making and exposing 

 the plates. Data are given in the column of remarks, from which 

 the reader can judge of the hardships to which the plates are often 

 exposed in the intervals — during sunless days and at night — and 

 these will be referred to later on. 



In column 6 we see how long each plate was exposed to the direct 

 rays of the. clear sun, and it is only necessary to add that four 

 hours of really bright sunlight is usually quite sufficient; indeed, as 

 reference to the previous communication shows (p. 394), excellent 

 results have been got with exposures of two hours to a December 

 sun. 



In column 7 are given the number of days that the plates were 

 incubated at 20° C. after exposure. In those cases where a positive 

 result was obtained, the time when the letter was distinctly visible is 

 recorded : where no letter, and therefore no bactericidal action, was 

 observed, the plates were incubated until the growth of the anthrax 

 was thick and decisive. 



Then comes the column summarising the results, and showing at 

 the same time what letter was employed. It only remains to point 

 out that no bactericidal action was detected behind the screens of 

 experiments No. 2 (olive), No. 4 (ruby), No. 6 (ruby), No. 7 (olive), 

 No. 9 (orange), No. 11 (ruby), and No. 12 (green), and if we com- 

 pare the data in Table I, this can be put as follows : — 



There is no perceptible bactericidal action behind any of the glass 



