ANNIVERSARY ADDRESS. 9 
forth in his paper the many difficulties which surround the problem 
—a problemwhich, involving the consideration of somany apparent 
anomalies, seems to suggest the need for re-examination of some 
of the accepted theories in physics. Most of us are familiar 
with the small glass vacuum bulbs enclosing the rotating discs, 
which are shown in the windows of philosophical instrument 
makers, and no doubt many have noticed the greater rapidity of 
rotation when much light is thrown upon them than when there 
is little light. When the instrument was first invented it was 
thought to be conclusively shown that the rotation resulted from 
a hitherto unknown mode of action of light. The fact that the 
light of a candle would cause a radiometer to rotate, even when 
the light was passed through a solution of alum, which is 
believed to stop all radiant heat, was put forward as a triumphant 
proof that an important discovery with regard to light had been 
made. A multitude of experiments were, however, soon brought 
to demonstrate that heat unaccompanied by light would pro- 
duce the same results. The theory of the mechanical action of 
light had therefore to be given up, and many and various experi- 
ments were made to solve the question of “ repulsion resulting 
from radiation.” Mr. Stoney gave the results of his experi- 
ments to the Royal Society in connection with the question ; 
and subsequently Mr. Crookes, in April, 1877, exhibited to the 
same Society a form of radiometer differing so far from the 
ordinary instrument that he proposed to give to it the name of 
“‘otheoscope.” In the radiometer the alternate sides of the 
dises of the fly are bright and dark; in the otheoscope the 
heater is stationary, and the cooler rotates. In the radiometer 
_the glass bulb is an essential part of the machinery, for without 
it the fly would not move. In the otheoscope Mr. Crookes 
believes it is only uscful to preserve the requisite amount of 
rarefaction. The unsolved mysteries which still enshroud it give 
to the radiometer an interest second to none amongst the scien- 
tific problems of the present day. 
I dare say most of you may remember the visit of 
H.M.S. ‘‘Challenger”. to this port, and some of you may 
i 
