THE RESISTING FORCE ON A SPHERE MOVING ROUND THE SUN AND RADIATING PRESSURE OF LIGHT I HOW LIGHT EXERTS PRESSURE WHEN we see the havoc wrought on a sea-wall by a storm, it is easy to believe that ocean waves exert a pressure against the shore on which they beat. But it is not easy to think that the tiny ripples of light also press against every body on which they fall, to think that when a lamp is lighted waves of pressure are sent out from itpressing against the source from which they start, pressing against every surface which they illuminate. Yet we now know certainly that light does exercise such pressure. It is a very minute pressure, far too small, even when it is strongest, to be felt by our bodies, and only to be detected by exceedingly sensitive apparatus. In the following pages I shall try to give some account of the reasoning by which the existence of light-pressure was predicted, and shall then describe the experiments by which it was, many years later, actually detected and measured. I shall then point out some consequences of the pressure which may hereafter be verified by astronomical observation. A hundred years ago it would have been much easier to explain how light exercises pressure than it is to-day. Then almost every one believed that FIG. I. light consisted of corpuscles, inconceivably small bodies, darting out at enormous speed from every glowing surface. Each molecule or atom of the surface was supposed to be a little battery of guns, keeping up a continuous fire of shot, each shot immensely smaller than the atom which fired it off. Every surface exposed to the light was regarded as |