Photographie Action of Radium Rays. 289 
presented to the radium for each period of time. It was 
then developed in alkaline hydroquinone for three minutes, 
and the intensities of the developed images were compared 
by means of a commercial densitometer. A similar experi- 
ment was also made with 50 milligrammes of radium 
bromide. For most of the experiments a red label rapid 
Ilford plate was used, and in a few a ‘ Lumiere’ plate. 
The general result of these experiments showed that the 
intensity of the developed image increased rapidly to a 
maximum value, then decreased rapidly, and finally the 
intensity decreased very slowly until a stage was reached in 
which there was practically no dark image formed on 
development. The time of exposure required to reach the 
maximum depended on the mass of the radium bromide used, 
that with 50 mgr. being earlier than with 10 mgr. In fact, 
after 40 hours’ exposure the effect on the plate might be 
compared to the complete reversal of image obtained when a 
plate is very much over-exposed to a bright light. The 
following curves refer to Ilford red label rapid plates, 
I. exposed respectively to 50 mgr. and IJ. to 10 mgr. 
(fig. 1). With this class of plate the maximum was reached 
Pie i 


0 10 20 30 . 40 50 60 
MINUTES 
in 15 minutes with 10 mgr. of radium bromide, and in 
7 or 8 minutes with the 50 mgr. With Lumiére plates the 
maximum was reached in about 2 hours, see fig. 2 (p. 290). 
To verify the result that an exposure greater than a certain 
critical value will produce a less dense image, the following 
experiment was made. A slit was cut in a thick sheet of 
lead and a covered photographic plate was placed below it. 
It was then exposed to radium rays for 10 minutes. Then 
the lead plate was removed, and the plate was exposed for 
a further period of 10 minutes. In this way the parts 
opposite the open slit received altogether 20 minutes’ 



