475 
1908-9.] A Simple Radioscope and a Radiometer. 
slightly lower temperature, and therefore not so great as the first ; but this 
does not seem appreciably to affect the results. After a few compressions 
and expansions things get to a uniform condition, especially if the com- 
pressions and expansions are made at equal intervals of time. 
For illuminating the test chamber B the lantern N is used. A common 
gas-jet aided with a lens is required for some experiments, but for most 
cases an incandescent gas light may be employed and the lens omitted, as 
without it one gets a more uniform illumination. These experiments are 
best made in a dark room, and only as much light allowed to come from 
the lantern as is necessary. A vertical adjustable slit placed in front of 
the lantern works well. This slit should only be opened about 1 cm. 
for tubes of 2 cm. diameter, as the narrow strip of light prevents the dis- 
agreeable and distracting reflections from the glass surfaces. When well 
arranged with black background the tube should show nearly black. A 
magnifying lens held in an adjustable support will be found useful. The 
lens should be set so as to look rather downwards, to avoid the reflections 
from the surface of the tube, and across but rather towards the light. 
The method of operating with the apparatus is as follows : — The U tube 
A, after being thoroughly cleaned, is filled with water to about the amount 
shown in fig. 1. For this purpose a fine india-rubber tube, small enough to 
pass through the opening in the U tube, will be found useful ; by sucking 
out the air it is quickly filled with water, and by blowing in air it is rapidly 
emptied. The tube, after washing and filling, is connected by means of the 
tube C with the compressor D. The light is then adjusted to show the 
interior of the tube, and as little light reflected by the glass as possible. 
The lever G is now pressed down for a short time and released ; and on now 
examining the test chamber B it will be found to be full of fog formed by 
the vapour condensing on the dust in the air. The magnifying lens should 
now be adjusted so as to show each fog particle clearly. A number of 
compressions and expansions are next made by pressing and releasing 
the lever G. This must be done till no fog particles appear in B ; the 
last of them will be seen falling like small rain-drops. After all con- 
densation ceases on slow expansion, the screw I is loosened and the stop J 
slipped up to a point which will require a little compression for the catch 
L to engage with the stop J. The lever is then pressed down till L engages 
with J. After a short time the spring K is drawn back and the catch L 
released, when the lever at once springs up and the air in B is suddenly 
expanded. The air in B should be watched while this is taking place. If 
no drops appear, then the expansion has not been enough to give the 
supersaturation necessary to cause condensation on the nuclei present. If 
