134 REPORTS ON THE STATE OF SCIENCE. 
A discharge tube of the type usually employed by the writer is 
closed by a fluorite window, and upon this window is mounted an air- 
tight ‘ screen chamber ’ 1 cm. thick, also closed by a fluorite window, 
which communicates with an air-pump. Above the ‘ screen chamber ’ 
is the air-tight condenser chamber, which may also be exhausted. 
This last chamber contains a clean zinc plate, 9 mm. in diameter, aud 
below it a brass guard ring. The apparatus is so constructed that the 
light from the discharge tube, after passing through the screen chamber, 
falls directly on the zine plate; the brass ring is protected from illu- 
mination and is earthed. The zine plate is connected to a gold-leaf 
electrometer of the simplest type, and is given a negative charge. The 
discharge tube is filled with a mixture of hydrogen and carbon monoxide, 
at about 2 mm. pressure, and the condenser chamber is exhausted. 
Now, if the screen chamber is filled with air at 1 atmosphere pressure, 
on starting the discharge tube the leaves of the electroscope collapse 
slowly. On the other hand, if the screen chamber is exhausted to about 
1 mm. pressure, on starting the discharge tube the leaves of the electro- 
scope collapse almost instantly. The phenomenon is so striking that the 
simplest electric arrangements serve to show it. 
It is known from spectroscopic measurements that the spectrum of 
a mixture of hydrogen and carbon monoxide consists of a great number 
of lines and bands extending from the region of the visible to the limit of 
the spectrum as set by fluorite— 1250. It is also known that a layer 
of air, at atmospheric pressure, 1 cm. thick, absorbs all wave lengths 
shorter than A 1750 very strongly, and that a layer of air at 1 mm. 
pressure, 1 cm. thick, shows little absorption until the region of » 1300 
is reached. Thus when the ‘‘ screen cell ’’ is filled with air, light from 
the red end of the spectrum—near \ 6800 to A 1750—falls upon the 
zine plate; when the cell is exhausted light from the visible to % 13800 
falls on the plate. The addition of the region between \ 1750 and 
A 1300 increases the leak by at least tenfold. Thus it seems probable 
that the photo-electric effect for zinc increases rapidly with decrease in 
wave length as the limit of the Schumann region is passed. It is the 
rapidity of this increase which is the striking point. 
In the volume ionisation of gases the same state of things holds true. 
Lenard investigated the phenomenon, and lately Professor Sir J. J. 
Thomson has proved its reality. The amounts of ionisation obtained 
were, relatively speaking, small. It appears, however, that if care is 
taken to employ light of the shortest wave length, the ionisation obtained 
is quite considerable.! Here again the rapidity of the increase of the 
effect as the Schumann region is entered is the important point. 
This phenomenon of volume ionisation in its relation to short wave 
lengths has some bearing on the behaviour of vacuum tubes, and 
accounts for a part of the effects usually attributed to ‘ Entladung 
strahlen.’ ? 
Such, in brief, are the results which have been obtained in this small 
but interesting spectral region. A complete description of the apparatus 
which has been used and the methods which have been employed is to 
be found in the articles to which references have been made. 
' Astrophysical Journal, vol. xxviii. No. 1, p. 56; Nature, April 23, 1908, p, 582. 
2 Astrophysical Journal, vol. xxviii. No. 1. 
