When we look at the sun, or any luminous body, through the common coloured glasses, the transmitted light, though tinged with one colour, nevertheless transmits rays of all the other colours, as may be proved by decomposing it with a prism. It was observed, however, by M. Monge, and the observation has been subsequently confirmed by . assenfratz and M. Arago, that the glass of old churches which has been stained either red or green by the oxide of copper, has the surprising property of transmitting no thing but the homogeneous green or the homogeneous red rays. This property will be of the greatest use in solar observations, as it will remove completely the imperfec tions of telescopes arising from their different refrangi bilities. A telescope should be constructed with a com pound object glass, to destroy as much as possible the aberration of sphericity. The red or green glass will remove all the heterogeneous rays, and the most perfect image of the sun will thus be obtained.
A series of new experiments have recently been made upon glass by Dr Brewster, and the results which he has obtained are of such a singular nature, as to lay the foun dation of a new science, analogous in its general character to the sciences of electricity and magnetism. He has shewn that when radiant heat is propagated along a plate of annealed glass, its progress may be rendered visible by exposing it to polarised light, a series of beautiful colour ed fringes advancing along the glass. The opposite edge of the glass, however, where the radiant heat does not exist in a sensible state, exhibits the same fringes, and conse quently it follows, that in its propagation along glass, radiant heat possesses the singular property of altering the structure or the mechanical condition of those parts of the glass where it does not exist in a sensible state. When the
heat is uniformly diffused over the plate of glass, all the coloured fringes vanish. By a particular process which we have not time to describe, he has succeeded also in communicating a permanent structure to glass, similar to that which it possesses during the propagation of radiant heat. The pieces of glass that have been subjected to this process, exhibit, by exposure to polarised light, the most brilliant and varied colours, arranged in the finest geome trical forms, and infinitely superior, in point of beauty, to any analogous production of art.
These plates of glass have exactly the same relative action upon the particles of light, as a magnet has upon particles of iron. The glass has a polarity as distinct as that of the magnet, and a piece cut from one pole of the glass ac quires a new polarity, exactly similar to what takes place by cutting off a part of a magnet. The results which have now been mentioned, lead also to the construction of a chromatic thermometer, which measures all differences of temperature, up to the melting point of the glass which is employed in its construction. A full account of these ex periments will be found in our articles OPTICS, POLARISA 1 I ON, and THERMOMETER. FOP an account of the sounds produced by glass, see HARMONICA.