The polarization of light, as it has been unfortunately called--unfortunately, as conveying an idea of determinate and different points or poles, which only exists in hypothetical analogy--presents to us a class of phenomena which promise to unclose the mysterious doors of the molecular constitution of bodies. This remarkable condition, as produced by the reflection of light from glass at a particular angle, was first observed by Malus, in 1808,[99] when amusing himself by looking at the beams of the setting sun, reflected from the windows of the Luxembourg Palace through a double refracting prism. He observed that when the prism was in one position, the windows with their golden rays were visible; but that turned round a quarter of a circle from that position, the reflected rays disappeared although the windows were still seen. The phenomenon of double refraction was noticed, in the first instance, by Erasmus Bartholin, in Iceland-spar, a crystal the primary form of which is a rhombohedron; who perceived that the two images produced by this body were not in the same physical conditions.[100] It was also studied by Huyghens and Sir Isaac Newton, and to our countryman we owe the singular idea that a ray of light emerging from such a crystal has _sides_. This breaking up of the beam of light into two,--which is shown by looking through a pin-hole on a card through a crystal of Iceland spar, when two holes become visible, is due to the different states of tension in which the different layers constituting the crystal exist. In thus separating the ray of light into two rays, the condition called polarisation has been produced, and by experiment we discover that the single ray has properties different from those of the compound or ordinary ray. It is somewhat difficult to explain what is meant by, and what are the conditions of, _polarised light_. In the first instance let us see by what methods this peculiar state may be brought about. If we reflect a ray of light from the surface of any body, fluid or solid, but not metallic, at an angle between 53° and 68° it undergoes what has been called _plane polarisation_. It may also be produced by the refraction of light from several refracting surfaces acting upon the pencil of light in succession; as by a bundle of plates of glass. Each surface polarises a portion of the pencil, and the number of plates necessary to polarise a whole beam depends upon the intensity of the beam and the angle of incidence. Thus, the light of a wax candle is wholly polarised by forty-seven plates of glass at an angle of 40° 41'; while at an angle of 79° 11' it is polarised by eight plates. Again, plane polarisation may be produced by the double refraction of crystals. Each of the two pencils is polarised, like light reflected from glass at an angle of 56° 45', but in opposite planes.