Page:Astronomy for Everybody.djvu/310

schein is simply this tail seen endwise. This is not an impossibility, but there is no proof that it is true.

Facts are now being discovered, and physical theories developed, the ultimate outcome of which may be an explanation of a number of mysterious phenomena associated with the earth and the universe. These phenomena are presented by the corona of the sun, the tails of comets, the aurora, terrestrial magnetism and its variations, nebulae, the Gegenschein, and the zodiacal light. The theories in question belong rather to the physicist than the astronomer, and the writer does not feel competent to explain them fully in their latest form, nor to define where established facts end and speculation begins. He must therefore limit himself to a few points.

First in order we have a pressure exerted by light, which was pointed out by Maxwell thirty years ago, but which seems to have been very generally overlooked, by astronomers at least. This principle was deduced by Maxwell from the electro-magnetic theory of light, and may be stated as follows:

When a pencil of light impinges perpendicularly on an opaque object, it produces a pressure upon the surface of that object, determined by the condition that if the object were set in motion with the velocity of light, and the force against it were kept up, the power required to keep up the pressure would be equal to that carried by the ray of light.

Another way of expressing the principle is this: Sup-