undergoing bombardment by the corpuscles, and it was quite natural to suppose that the surface was pressed back.

We may illustrate the supposed mode of action by fixing a vertical tin disc at the end of an arm and suspending the arm by a fine wire, so that it is free to turn round as in fig. I.

Now arrange a funnel and a metal pipe, as in

DISC

0

FIG. 2.

fig. 2, in which the disc is seen edgewise, and the suspension is not shown.

Pour some fine shot into the funnel, and they run down the pipe and bombard the disc, pressing it back. The shot acquire momentum. They carry this momentum and give it up to the disc when they hit it. This giving up of momentum is pressure.

In the eighteenth century, when the Corpuscular

Theory of Light flourished, many experiments were made to detect a pressure by allowing light to fall on a disc, like that in fig. 1, on a small scale, and very delicately suspended, sometimes in air, sometimes in a vacuum. Sometimes the disc was pressed back, sometimes it was drawn forward, and no observer obtained conclusive, or even consistent results.

If these early experimenters had known the Principle of the Conservation of Energy, they would have been able to calculate the value of the pressure, which they were looking for, and it would have worked out on their false theory to double the actual value we now know it to have. Even this double value is far too minute to be detected by the means then attainable.

Their variable results, now an attraction now a repulsion, were due, no doubt, to two actions which are still the terror of all experimenters on the subject. When they worked in air, the light absorbed by the disc heated it. The disc in turn heated the surrounding air, which expanded and streamed up bodily, forming currents known as "convection currents"-simply minute upward gales of wind. If a flat iron plate is heated and then held in front of a lantern these currents form faint shadows on the screen and may be

1 Note I, p. 83.

It depends entirely on

seen rising like smoke. the lie of the plate whether these rising streams of air will tend to press the plate back or to draw it forward. The action of the air currents on a disc heated by a beam of light may easily be many times greater than the pressure of

the light.

When they worked in a vacuum probably another action came into play, an action discovered and investigated by Sir William Crookes, who invented a beautiful little instrument to show it, which he named the Radiometer. radiometer in its commonest form consists of four small mica discs fixed at the four ends of a hori

The

zontal cross as in fig. 3. The cross is free to spin round on a pivot as frictionless as possible, and it is contained in a highly exhausted bulb about three inches in dia

FIG. 3.

meter. Each of the vanes is blackened on one side, and when a lighted match or candle is brought near the bulb, the blackened faces retreat from the source of light while the unblackened faces move towards it. At first it was supposed that the action might be directly due to the pressure of light, but it is easy to see that this pressure would produce

just the opposite motion. For the light falling on the unblackened surfaces is partly reflected and would therefore press not only in falling on the surface but would give, as it were, another kick back on being reflected, while the light falling on the blackened surface would press only in falling on it, for it is absorbed there and does not rebound. The unblackened surfaces would therefore retreat.

It was soon found that the action of the radiometer is due to the residual air still remaining in what we call the vacuum in the bulb. The blackened surfaces absorb the light and so become hotter than the unblackened surfaces. The molecules of air in the bulb are rushing about in all directions, and those which hit the hot black surface get a little extra energy from it and go back faster than they came up, and so give a greater kick back against it than if they rebounded at the same speed. Those which come up on the other cooler side go back with the same velocity with which they came up and do not give an increased kick back. Thus the residual air presses more against the black surface and the vanes spin round.

For reasons which we cannot enter into here, this "radiometer action," as it is termed, only comes into serious consideration when the air is very much rarefied. But no doubt it was present in the early attempts made to detect light-pressure on a disc in an exhausted vessel.

Convection currents, then, disturb the experiments when we work in air, and radiometer action disturbs them when we work in a vacuum. We shall see later how it is possible to steer between Scylla and Charybdis and reveal the true pressure due to light.

It is just a hundred years since Thomas Young killed the corpuscular theory of light and founded in its place the theory that light consists of waves, a theory soon accepted by every one. But there was no reason at that time to suppose that the waves could press, and so experiments to detect light-pressure ceased for nearly a century.

In 1873 Clerk Maxwell put forth the Electromagnetic Theory of Light, a theory now universally accepted. On this theory light still consists of waves, waves of electric and magnetic disturbance just like the waves used in wireless telegraphy, but microscopic in length instead of being yards or miles from crest to crest. He showed, too, that such waves should exert a pressure, but just half that exerted according to the discarded Corpuscular Theory. He calculated with the data he took that strong sunlight falling perpendicularly against a black surface exerts a pressure of rather less than one two-hundred-thousandth of a grain on a square inch, or rather less than one twentythousandth of a milligramme on a square centimeter. It only amounts to two and a half pounds weight on a square mile.