if the second plane of incidence be at right angles to the former, then no part of that ray will be reflected by the second surface; and an eye rightly placed for witnessing the effects, will perceive in the exact specific direction a central spot of absolute blackness, surrounded also by a dark space of some extent, from which less or more of light is reflected, in proportion to the increase of distance from the central line of no reflection. The light, however, which has thus been polarized, may be wholly, or in part, depolarized by the interposition of many crystallized bodies, the degree of depolarization being dependent on the more or less exact position of a certain neutral axis or plane of their crystalline texture with the plane of primitive polarization. When these planes are perfectly coincident, the light remains polarized in that plane, and a black line appears in that direction; but adjacent to it, on each side, are seen a series of colours, which depend partly upon their proximity to the central black line, and partly upon the thickness of the depolarizing body, the succession of colours being exactly the same as those observed by Newton in thin plates, but variously modified in their forms, according to the nature of the crystalline substance interposed, and according to the position of its axis.
The optical effects of heated glass, as now observed by Dr. Brewster, are precisely of the same kind, and are now found to depend not upon the simple circumstance of temperature of the entire plate, as he originally supposed, but upon the progressive differences of temperature in different parts of the plate, arising from contact or proximity to a plate of hot or cold iron, or from the cooling power of the surrounding atmosphere. And in the same manner as the several tints of colour produced by crystallized bodies, have been shown by M. Biot to depend on a series of thicknesses proportional to those in Newton's scale for thin plates; so with respect to heated glass, Dr. Brewster observes, that a corresponding arithmetical progression is observable for the same tints, whether the thicknesses compared be those of single plates, or the aggregate thickness of several combined.
From these phenomena, Dr. Brewster infers the production of what he calls a crystalline structure in the glass during its contact with heated iron; but observing the existence of an opposite structure in the middle of the glass, and again the same structure at its remote extremity apparently beyond the reach of sensible heat, he says there is nothing analogous but in the perplexing phenomena of magnetic and electric polarity.
In the prosecution of these experiments, the author varies indefinitely the forms and dimensions of his plates of glass, and with them the forms of the fringes produced; but it would be next to impossible to convey any correct idea of the various appearances without assistance from drawings; neither indeed could Dr. Brewster himself have observed the phenomena with sufficient precision in their fluctuating state as arising from the temporary communication of heat, had he not found means to render the same properties permanent,
