1898 - 99 .] Rev. A. Moffat on Energy of the Bontgen Bays. 433 
uniformly illuminated surfaces, which can be readily compared 
when the sources of light are strong enough. I found that the 
light emitted by the paper was one-twentieth of that of the Hefner 
lamp. I then removed the paper, and determined the extinction 
coefficient of the green glass and the darker glass by measuring 
the intensity of the light transmitted by them in comparison with 
that of the lamp alone. I used four different pieces of dark glass 
(A, B, C, D) in my experiments, and found that the intensity of 
the light coming from the translucent paper, when weakened by 
transmission through the green glass, and one of these pieces of 
dark glass was, 
When A was used, -goVo Hefner lamp. 
T> 1 
>5 1 1,000 5J 5’ 
P 1_ 
JJ ^ 5} 16,000 55 55 
T) 1 
55 ^ 55 50,000 55 ” 
When we seek to compare the energy of the Rontgen rays with 
that of light rays, we must determine how much energy is supplied 
by them in equal times. Now, the light of the Hefner lamp is 
emitted during the whole time it is burning, but the Rontgen rays 
only during very short intervals of time. I therefore tried to get 
at least an upper limit to the duration of these intervals. This 
I did by putting in front of the barium platino-cyanide screen 
an opaque screen with a small vertical slit, and viewing the image 
of the slit in a rotating mirror. The mirror was driven by a 
centrifugal machine, and the image viewed directly by the eye. 
Now, since the barium platino-cyanide screen does not show any 
appreciable phosphorescence, it follows that if the emission of the 
Rontgen rays continues for a certain time, we should see a broaden- 
ing of the image in the mirror.* Had there been a broadening of 
one-tenth of the image, it could have been detected ; but no such 
broadening was observed. Now, since the slit was 3 mm. broad, 
and the mirror exactly opposite it at a distance of 27 '5 cm., and 
revolving at the rate of about ten revolutions per second, it follows 
* It is clear that, if the emission of light from the screen continued after 
the Rontgen rays had ceased to fall upon it, we could not use the above- 
mentioned method, as there would be a broadening of the image, owing to 
this phosphorescence. This would be the case if barium platino-cyanide phos- 
phoresced, e.g., like the uranium salts or Balmain’s luminescent paint. 
