EMISSION VELOCITIES OF PHOTO-ELECTRONS. 
209 
passed through the monochromator was a very small fraction of that emitted by the 
lamp. 
Some anomalous results in the earlier experiments led to the discovery that the 
monochromator did not isolate the wave-lengths perfectly. It was set so as to isolate 
one wave-length, and the light which emerged was analysed by means of a second 
quartz spectrograph. It was found that, in addition to the isolated line, all the other 
wave-lengths were present though much weaker. This effect leads to spurious results 
when working with the longer wave-lengths which are made to appear photo- 
electrically active when they are not. This error only becomes appreciable when 
using wave-lengths longer than X 3000. Sheets of mica of different qualities were 
found to cut off wave-lengths shorter than X 2967 and X 3125, thick glass transmitted 
down to X 3340, and a solution of chromium chloride absorbed all wave-lengths shorter 
than X 3665. During the course of the investigation, a new wave-length X 1849 in 
the mercury arc spectrum was discovered.* 
5. The A'ppciratus. —The amount of energy which is transmitted by the monochro¬ 
mator when set to isolate any one wave-length is exceedingly small. The small 
photo-electric effect which would result from this feebleness in the light determined 
that great sensitiveness to electrical changes must be an important feature of the 
apparatus; Hence it is that, in the apparatus which was finally adopted, the 
electrical capacity is very small. The apparatus is shown approximately to scale in 
fig. 2. 
The plate on which the light is incident is a disc, N, of nickel, 3 cm. in diameter. 
The disc is suspended by a thin wire from a slide, S, which runs in grooves in the 
tube T. The disc and wire are insulated from S by the quartz rod Q. This system 
can be raised or lowered by means of the winch, E, which is a glass tap with a groove 
cut round the barrel. S is suspended from the winch by a silk thread. The metal to 
be distilled is heated in a little furnace, F. This is a quartz-ignition spoon into which 
fits a thin quartz cup containing the metal. Between the cup and the quartz spoon 
there is a thin sheet of Pt or Ni, which can be heated to redness by a current. As 
the vacuum forms a good heat insulator, the metals can be raised to a liigh tempera¬ 
ture. To get rid of occluded gases the quartz furnace containing the metal is heated 
for a long time in an auxiliary vacuum, until little or no gas is evolved during distillation. 
In this way surfaces of Ca, Bi, Sb, Zn, Cd, Se, Pb, Mg, and As were deposited on 
the disc, the whole process being carried out in a liquid air vacuum. When the 
distillation is finished the disc is raised to the position shown in the diagram. 
Contact with the tilted electroscope is made by means of a spring. S', which rests on 
a short cross wire behind the disc. The electroscope is placed close to the apparatus. 
To keep the capacity as low as possil^le thin connecting wires are used. The capacity 
of the insulated system is probably below 10 cm. 
VOL. CCXII.-A. 
* Hughes, ‘Proc. Camb. Phil. Soc.,’ XVL, p. 428, 1912. 
2 E 
