432 
Proceedings of Royal Society of Edinburgh. [sess° 
with a small hole in it, and through this the light-emitting surface 
of the screen appeared. In order to stop the discharge in the 
Rontgen lamp when I wished to do so without stopping the motor, 
I introduced a shunt, consisting of an air-spark, parallel to the 
lamp. 
A Hefner lamp was used as the standard of comparison. As 
preliminary experiments showed that the light proceeding from the 
barium platino-cyanide screen was very faint, the light coming from 
the Hefner lamp had to be weakened considerably. This was done 
by putting the lamp inside a wooden box, provided with tubes for 
the entrance of fresh air and the escape of the combustion products. 
A hole was made in front of the box, and covered with translucent 
paper of as uniform texture as could be procured. In front of 
this, again, was put a piece of green glass, and as this did not 
weaken the light enough, another piece of somewhat darker smoked 
glass was put in front of it. The Hefner lamp was placed sufficiently 
far behind the paper to secure that it was practically uniformly 
illuminated, and as in the case of the barium platino-cyanide screen, 
a piece of cardboard with a small hole in it was set in front to 
give the light-emitting surface a convenient magnitude. 
In the choice of the photometer to be employed, I was restricted 
by the very small luminosity to be measured. I could not, there- 
fore, adopt either Joly’s paraffin photometer or the photometer of 
Lummer and Brodhun. I used simply two 
mirrors set at right angles to one another, and 
in this way lost very little of the light. At the 
barium platino-cyanide screen and the Hefner 
lamp, I used apertures of different sizes, vary- 
ing from 5 cm. to 1*5 cm. in diameter, in order 
that the images seen in the photometer might 
appear about the same size in whatever position on the optical bank 
it might happen to be. 
To determine the quantity of light emitted by the translucent 
paper, which served as the source of light when illuminated by the 
Hefner lamp, I put at one end of the optical bank a Hefner lamp, 
and, at the other end, the box with the translucent paper thus 
illuminated. The photometer I now used was that of E. W. 
Lehmann, described in Wied. Ann., Bd. 49, p. 672, as it gives two 
