720 
DR. ARTHUR SCHUSTER ON THE FORCE PRODUCING THE 
I have shown that an exterior force of 5 per cent, the amount of the interior force 
could have been detected. There cannot exist, therefore, an exterior force exerting a 
greater pressure on the unit of area of the light-mill than the 500th part of a milligram. 
According to Prof. Maxwell’s theory of light, an exterior force should exist ; but 
this force would be smaller than the limits which we have given as having an appre- 
ciable effect on the experiments. By concentrating the rays of solar light in the ratio 
of one square foot to a square centimetre, however, the pressure on that square centi- 
metre would be equal to 0-00004 gramme, a quantity which could not escape obser- 
vation. 
I think that the experiments described in the foregoing pages are interesting, not 
only by showing that the forces investigated by Mr. Ceookes are interior forces ; but 
also by giving a method by means of which we are able to distinguish between such 
interior forces and forces directly referable to radiation. We may thus hope to find 
out whether such a pressure as that indicated by Maxwell’s electromagnetic theory of 
light really exists. 
Appendix (added Oct. 29, 1876). 
It was suggested by Prof. Maxwell to compare the motion of the glass vessel 
containing the radiometer with its equations of motion as deduced by him. I have 
therefore made a set of more careful experiments, and calculated from them, according 
to Prof. Maxwell’s equations, the intensity of the force which acted on the vanes of 
the radiometer during my experiments. The number obtained has of course no other 
value than that of giving us an idea of the approximate magnitude of the force, as we 
have no accurate knowledge of the intensity of radiation actually falling on the vanes. 
My measurements show, however, that the motion of the vessel can be represented 
satisfactorily by a simple formula. 
In order to deduce the equations of motion we shall, to simplify the calculation, 
make certain assumptions known to be only approximately correct. The first of them 
is, that the force acts with constant intensity during the experiments. The second is, 
that the resisting stress is proportional to the difference in the velocities of the radio- 
meter and the vessel. 
Let I and i be the moments of inertia of the glass vessel and of the light-mill. 
Let y and x be their angular displacement. 
Let x, be the coefficient of damping between the mill and the vessel. 
Let K be the coefficient of damping between the enclosure of the light-mill and the 
outer vessel in which this enclosure is suspended. 
Then if H y is the force of restitution due to the bifilar suspension, the equations 
of motion are : — 
1. For the mill 
( 1 ) 
