Dixon & "Wigham — Radiations from Radium Bromide. 181 



subsequently arranged our experiments so that the cultures to be 

 exposed to the radium bromide were, at one point at least, less 

 than this distance from the radium, and only separated from it by 

 the glass of the containing tube and a small thickness of air. 



The figures in the accompanying Plates are reproductions of 

 contact photographs of agar -plate-cultures which had been exposed 

 in this manner. 



Fig. 1, Plate XVI., is a photograph of an emulsion of B. 

 pyocyaneus spread on a glass plate. This culture was exposed 

 to the radiations from the radium bromide at a distance of 10 mm. 

 from the tube for four days, the tube being supported on a tiny 

 wire stand over the central region of the plate. During these 

 days the bacteria did not develop, as the temperature was low. 

 On the fifth day the culture, still exposed to the radium-tube, 

 was transferred to an oven at 22° C, and incubated for three days, 

 when the photograph was taken. The central region, which was 

 closest to the radium, may be seen bare of colonies ; while the 

 bacteria in those parts which were more than 15 mm. removed 

 from it have developed normally. 



Fig. 2, Plate XVI., is from a similar but denser emulsion of 

 B. pyocyaneus. This culture was exposed for four days in the cold 

 to the radiations at a distance of 7 mm. from the radium bromide. 

 After incubation for two days at 22° C. the photograph was taken. 

 The sterile region is here also very well marked. The line of 

 denser growth seen below the clear patch in the figure is due to 

 the screening action of a platinum rod 3 mm. in diameter, which 

 was supported between the radium-tube and the culture. In a 

 photograph taken the day before (not reproduced here), the sterile 

 patch was of greater extent than appears in this figure, and the 

 line of dense growth stood out prominently across the greater part 

 of the plate. The delay in development of the colonies on the 

 margin of the sterile patch is of interest in showing that the 

 radiations may act as a retarding influence on growth, even where 

 they cannot inhibit it altogether. 



After the photograph which is reproduced in fig. 2 was taken, 

 the radium was removed from the culture, and the culture-plate 

 was transferred to an oven at 37° 0. Figs. 1 and 2, Plate XVII., 

 reproduce photographs which were taken after incubation at this 

 temperature for two and six days respectively. It may be noted 



SCIENT. PItOC. R.D.S., VOL. X., PART II. P 



