Page:Optics.djvu/202

 will be observed on the second glass. To exhibit them in a methodical manner, which will allow us easily to take them all in at one view, we will suppose, as above, that $$STL$$ the plane of incidence of the light on the first glass coincides with that of the meridian, and that the reflected ray $$I I'$$ is vertical. Then if the collar $$T'T'$$ which bears the second glass be turned round, this glass will also turn all round the reflected ray, making always the same angle with it, and the second reflexion will be directed successively to all the different points of the horizon: this being premised, the phænomena that will be observed are as follows:

When the second or lower glass is placed so that the second reflexion takes place in the plane of the meridian like the first, the intensity of the light finally reflected is at its maximum. As this glass is turned round it reflects less and less of the light thrown on it.

Finally, when the lower glass faces the east or west point, the light passes altogether through it without being reflected at either surface.

If the collar be turned still farther round, the same phænomena recur in an inverse order, that is, the intensity of the light reflected increases by the same degrees as it before diminished, and attains the same maximum state when directed towards the meridian, and so on through the whole circle.

It appears then, that during a whole revolution of the glass the intensity of the reflected light has two maxima answering to the azimuths 0 and 180°, and two minima answering to 90° and 270°. Moreover, the variations are quite similar on different sides of these positions. These conditions will be completely satisfied by supposing, as Malus does, that the intensity varies as the square of the cosine of the angle between the first and second planes of reflexion.

The results of this interesting observation being thus collected into one point of view, we may draw this general consequence from them, that a ray reflected by the first surface is not reflected by the second, (under a particular incidence) when it presents its east or west side to the surface, but that in all other positions it is more or less reflected. Now if light be a matter emitted, a ray of light can be nothing else but the rapid succession of a series of