716 
DR. ARTHUR SCHUSTER ON THE FORCE PRODUCING THE 
end of the “ light-mill,” which was suspended in this way, was dipped into a small beaker, 
tilled with oil, in order to steady the motion of the vessel and to bring it soon to its 
position of rest. The azimuth of the suspended vessel was read otf on a scale by means 
of a dot of light concentrated by the concave mirror attached to the vessel. The time 
of vibration of the vessel was found to be about twenty-two seconds for a complete 
oscillation. The logarithmic decrement was found to be about 0T76. 
The beam of light of an oxyhydrogen lamp was concentrated by means of a lens, and 
the vessel was placed at such a distance from the focus that the cone of light just enclosed 
the whole of the revolving mill. In this position the instrument showed the greatest 
sensitiveness. The “ light-mill ” revolved about 200 times in one minute. 
The light was cut off at the beginning of the experiment by means of a screen, and 
the position of rest of the glass vessel was read otf by means of the dot of light on the 
scale. The screen was then suddenly removed, and in every case a large deflection of 
the glass vessel containing the light-mill was observed. The vessel was deflected in a 
direction opposite to that in which the mill turned. When the velocity of the revolving 
mill had become constant, the outer vessel gradually came back to its original position 
of rest. This is a fact of great importance, and considerable care was taken to find out 
with what degree of accuracy it could be established. Owing to disturbances produced 
by various causes, the zero could never be determined within two or three divisions of 
the scale. The position of rest of the vessel when the mill was turning with constant 
velocity was always found to be within that distance from the original position of rest. 
When the vessel containing the rotating “ light-mill ” had come to rest, the screen 
was suddenly replaced between the light and the mill. The mill now gradually came 
to rest, and at the same time the vessel was driven away in the opposite direction to that 
in which it moved on starting the experiment. The vessel, on stopping the light, was 
turned in the same direction in which the light-mill revolved. I shall give the readings 
of three experiments taken out of a great many made at different times and with different 
intensity of light. 
Position of rest (light cut off) . . 
f 
. I 
Light suddenly turned on : successive elongations of_j 
the vessels containing the light-mill . . . . | 
l 
Position of rest (mill turning with constant velocity) . 
Light suddenly cut off : enlongations of vessel . . j 
I. 
II. 
III. 
0 
0 
0 
+175 
+88 
+ 144 
- 95 
-31 
- 51 
+ 65 
+ 17 
+ 23 
- 43 
- 4 
- 11 
+ 29 
+ 7 
0 
+ 2 
0 
-137 
-62 
-127 
+ 79 
+ 28 
+ 43 
- 47 
- 8 
- 20 
