436 Proceedings of Royal Society of Edinburgh. [sess. 
d = distance in centimetres of the photometer from the 
barium platino-cyanide screen. 
e= Luminosity of the screen as compared with the Hefner 
lamp deduced from b, c , and d. 
a 
b 
c 
d 
e 
14 
A 
80 
118 
4x10-4 
13 
A 
97 
101 
2 „ 
12 
A 
105 
93 
1*6 „ 
11 
A 
120 
78 
0*8 „ 
10 
B 
105 
93 
0*7 „ 
9 
B 
116 
82 
0-5 ,, 
8 
C 
109 
89 
0*4 „ 
7 
C 
117 
81 
0*3 „ 
6 
D 
140 
58 
0-03 „ 
5 
very faint. 
The luminosity of the screen is thus about 4 x 10 " 4 Hefner lamp 
when the air-spark is 14 mm., and diminishes rapidly as the air- 
spark is shortened. 
In these experiments I have measured the intensity of the rays 
emitted by only one screen. I shall take it to correspond to the 
whole energy of the Rontgen rays, i.e ., as if the whole of the 
Rontgen rays were absorbed by the barium platino-cyanide screen. 
I shall do this because I found that when I put a second screen 
before the first, its brightness was only one-eighth of that of the 
first. The correction to he made for the Rontgen rays not trans- 
formed into light by the first screen would thus be only 10 per 
cent., a number which we may here neglect, as being of the 
order of the experimental errors. 
In order to compare directly the intensity of the Rontgen radia- 
tion with that of the Hefner lamp, we must hear in mind that the 
Rontgen rays come from the anti-cathode, and produce a lumi- 
nescence that proceeds in a diffused way in all directions from the 
luminescent screen. If, therefore, the distance of the anti-cathode 
from the screen is r , each part of the screen gets an intensity 
proportional to of the intensity of the whole radiation. How 
the radius of the bulb of the Rontgen lamp was 4*2 cm., and its 
distance from the screen 2 cm., so that the distance of the anti* 
