It was Huyghens that discovered the action of Iceland spar in doubly refracting light; and the light which passed the crystal was called _polarized light_ at the suggestion of Newton, who, it must be remembered, looked upon light as something actually emitted from luminous bodies; these projected particles were supposed, after passage through Iceland spar, to be furnished with poles analogous to the poles of a magnet, and to be unable to pass through certain bodies when the poles were not pointing in a certain direction. It was not until the year 1808 that Malus discovered the phenomenon of polarization by reflection. He was looking through a double-refracting prism at the windows of the Luxembourg Palace, on which were falling the rays of the setting sun. On turning the prism he noticed the ordinary and extraordinary images alternately become bright and dark. This phenomenon he at once saw was in close analogy to that which is observed when light is passed through Iceland spar. At first he thought it was the air that polarized the light, but subsequent experiments showed him that it was due to reflection from the glass. Let us examine some of the phenomena before we proceed to show the use astronomers make of them. It is the property of some crystals, such as tourmaline, when cut parallel to a given direction, called the optic axis of the crystal, to absorb all vibrations or resolved parts of vibrations perpendicular to this line, transmitting only vibrations parallel to it. A similar absorption of vibrations perpendicular to a given direction may be effected by various other combinations, of which one, Nicol’s prism, is in most common use. Any of these arrangements may be used as an analyzer with the telescope, for determining whether the light is completely or partially polarized, and in either of these cases which is the plane of polarization. The plane containing the direction of the rays and the line in the analyzer to which the transmitted vibrations are parallel, is called the plane of analyzation: all the light which reaches the eye consists of vibrations in the plane of analyzation. As we rotate the analyzer, we rotate equally the plane of analyzation. If we find a position of the plane of analyzation for which the light received by the eye is a maximum, we know that the light from the object is partially or completely polarized in a plane perpendicular to the plane of analyzation when in this position. To determine whether the polarization is partial or complete, we must turn the analyzer through an angle of 90° from this position: if we now obtain complete darkness, we know that there are no vibrations having a resolved part parallel to the plane of analyzation in this position, or that the light is completely polarized in this plane: if there be still some light visible, the polarization is only partial.