671 



Physics. — "A dlfevence between the action of light and of 

 X-raj/.s on the photographic plate". By Prof. I. K. A. Wektheim 

 Salomonson. 



(Communicated in the meeting of Septembei' 25, 1915). 



In a series of experiments on tiie quantitative action of A'-rays 

 and light on photographic plates, I found a characteristic difference 

 between the two kinds of rays. 



In these experiments so-called exposure-scales were made by 

 exposing one half of a plate to regularly increasing light-quantities 

 and the other half in the same way to Rontgen-rays. Both halves 

 were de\'eloped at the same time in one developing tray and also 

 fixed simultaneously in one tiay. 



On each of the negatives we find a series of small fields, which 

 have been exposed to the action of light or of .I'-rays of intensities 

 increasing in the ratio of 1, 2, 4, 8, 16 etc., and which show an 

 increasing density. On the half exposed to the X-ra_\s the time of 

 exposure and the hardness of the rays are also recorded. The 

 transparency of each of the small fields is photometrically measured. 

 The réciproques of the figures obtained in this way give the 

 absorption-factor, the logarithm of which is" the optical rfenóvVy. From 

 the figures for the density curves are drawn, the densities being 

 plotted as ordinates to the logarithms of the ex^josures as ordinates. 

 In this way we get the "characteristic curves" of the plates as used 

 by HuRTER and Driffield, Eder and others. 



The different precautions taken in these experiments need not l»e 

 described ; sufficient be it tliat the exposures, once started, were 

 automatically carried out, and that any irregularities in the intensity of 

 the light and the X-radiation either could bear no influence on the 

 result or could be inniiediately detected. 



Curves like these always show a curvature convex to the X-axis 

 corresponding to the underexposed part. The "correct exposures" 

 give a straight line. This part generally commences at a density 

 of roughly 0.5. The straight line prolonged to the axis of abscissae 

 meets it in the "point of inertia" (Beharrungspunkt) which is used 

 by HuRTER and Driffield to indicate the "speed" of the plate. It is 

 almost entirely independent of the time of development, the kind 

 of developer used and its temperature, which influence only the 

 slope of the curve in tlie straight part. We also know that the 

 quantity of silver in a negative increases proportionally to the 

 logarithm of the exposure. 



