28 



Prof. Marshall Ward. Experiments on the [Feb. lb', 



screens which transmit the red, orange, and yellow, but cut off the blue 

 and violet, rays. 



The green screen (No. 12) will have to be considered separately, 

 and remains doubtful for the present. 



On tbe other hand, distinct positive results were obtained in the 

 following experiments : — No. 1 (violet), No. 3 (blue), No. 5 (ordinary), 

 No. 8 (blue), and No. 10 (ordinary) ; in other words — 



The bactericidal action is marked behind the glass screens which 

 transmit most or all of the blue and violet rays, ivhether they cut off the 

 red, orange, and yellow rays or not. 



These results may be expressed still more shortly by saying that 

 the bactericidal action of the sun's rays is due to those in the blue- 

 violet half of the spectrum, a result perfectly in accordance with the 

 evidence obtained by working with the spectrum thrown on the plates. 



As is well known, however, it is possible to test such a generalisa- 

 tion by using screens of other kinds, and my experiments with these 

 are interesting, as showing not only the accuracy of the above con- 

 clusions, but also that the light is effective after passing through con- 

 siderable thicknesses of water. 



Experiments with Screens of Chemical Solutions.* 



T employed saturated solutions of ammoniated cupric oxide and of 

 potassium bichromate : as is well known, the first cuts off all the red, 

 orange, and yellow rays as far as the line b (in the green), while the 

 second cuts off the blue- violet from the line b onwards. 



On January 10, I made two plates of spores in agar, poured from 

 the same tube, and alike in all respects : they remained at 8 — 10° C. 

 till the 12th, when each was exposed. One was covered with a 

 stencil in which the letter T was cut, put behind a screen of am- 

 moniated cupric oxide, the glass faces of which measured each inch, 

 and the solution T 7 g- inch ; the other, with an exactly similar letter T, 

 behind a screen of potassium bichromate, measuring — inch in thick- 

 ness of each glass, and ^ inch of the solution. 



They were exposed to the south, side by side, on the 12th and 13th, 

 both dull days, with a gleam of sunshine for about one hour and two 

 hours respectively. A large concave mirror was also placed so as to 

 throw more light on each, as equally as possible. The temperature 

 rose to 20° C. occasionally, but not beyond. On the 14th they were 

 exposed for three hours to dull diffuse daylight, there being no 

 sunshine. 



* Such screens have been used, e.g., by Janowski (" Zur Biologie der Typhus- 

 bacillen," ' Centralbl. f. Bakt.,' vol. 8, 1890, p. 167), but both his methods and 

 results were different. It is inferred that the red rays are active, but his screen 

 (a solution of f'uchsin) probably allowed transmission of violet rays, as I have con- 

 vinced myself by sp^ctroscopical examination. 



