Non-scientific readers will still ask,--What is this mysterious condition of light which is produced by reflection and refraction at peculiar angles to the incident ray. It is one of the most difficult of problems to express in popular language. The conditions are, however, these:-- An ordinary ray of light will be reflected from a reflecting surface at whatever angle that surface may be placed in relation to the incident beam. A polarised ray of light is not reflected in all positions of the reflecting surface. An ordinary ray of light is freely transmitted through a transparent medium, as glass, in whatever position it may be placed relative to the source of light. A polarised ray of light is not transmitted in all the positions of the permeable medium. Supposing a plate of glass is presented at the angle 56° to a polarised ray, and the plane of incidence or reflexion is at right angles to the plane of polarisation of the ray, _no light is reflected_. If we turn the plate of glass round through 90°, when the plane of reflexion is parallel to that of polarisation _the light is reflected_. If we turn the plate round another 90°, so that the plane of reflexion and of polarisation are parallel to each other, again _no light is reflected_; and if we turn it through another 90° the reflection of the ray again takes place. Precisely the same result takes place when, instead of being reflected, the polarised ray is transmitted. Some substances have peculiar polarizing powers: _the tourmaline_ is a familiar example. If a slice of tourmaline is taken, and we look at a common pencil of light through it, we see it in whatever position we may place the transparent medium. If, however, we look at a pencil of polarised light, and turn the crystal round, it will be found that in two positions the light is stopped, and that in two other positions it passes freely through it to the eye. By way of endeavouring to conceive something of what may be the conditions which determine this very mysterious state, let us suppose each ray of light to vibrate in two planes at right angles to each other: one wave being vertical and the other horizontal. We have many examples of this compound motion. The mast of a ship, by the force with which she is urged through the water, describes a vertical wave, while by the roll of the billows across which she sails, a lateral undulation is produced at the same time. We may sometimes observe the same thing when a field of corn is agitated by a shifting wind on a gusty day. The hypothesis therefore is, that every ray of ordinary light consists of two rays vibrating in different planes; and that these rays, separated one from the other, have the physical conditions which we call _polarized_.