David Brewster

“ And since tangent is : radius :: sine : cosine, it is evident that the angle of refraction is then complement to the angle of incidence, or their sum is a right angle; and hence the reflected ray forms a right angle with the refracted ray.
At emergence also from a dense medium, the part reflected will be more or less polarized, and the maximum of polarization will be found as before, to occur when the angles of incidence and emergence are complementary to each other, or when the reflected portion makes a right angle with the refracted ray. ”

“ A circular plate of glass, cooled with its centre resting on a piece of cold iron, or a cylindrical rod of glass cooled in the open air, when examined by polarized light in the direction of its axis, each present the same appearance of a black cross through their centre, and concentric fringes parallel to their circumference. ”

“ Though Huygens, who first explained the laws that regulate the extraordinary refraction of light at a surface of Iceland spar, discovered that light thus separated has properties different from common light ”

“ Hence if other bodies, as hair, wool, silk, &c. have neutral axes or planes, in which a transmitted ray retains its polarization, while it is depolarized in other positions, this affords optical evidence of the regularity of their internal texture ”

“ Since considerable difficulty occurs in preserving unannealed drops of flint glass, the author has paid attention to the circumstances most favourable to their preservation, and recommends removing them from the water in which they have been plunged, as soon as the redness in the centre of the drop ceases to be visible. ”

“ Gum arabic depolarizes light in every position, unless in extremely thin chips. Cherry gum also depolarizes light, with some appearance of a neutral axis. Caoutchouc has the same property; and though it loses it when fused by heat, it recovers the same power gradually in the course of a few weeks. ”

“ In the author's concluding experiment, he formed one twentieth of an inch thick of the same jelly, by melting it between two plates of glass. When merely consolidated by cooling, this had no power of depolarization; but by pressure it instantly restored the evanescent image, and exhibited, as in the former cases, the complementary colours, showing, says the author, that pressure communicates a modification of structure correspondent to that of crystallized minerals. ”

“ 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, by a method which forms the subject of the second section of this paper.
When a plate of glass is brought to a full red heat, and is then cooled by placing its edge on a bar of cold iron, the same fringes of colour are developed during cooling as by placing cold glass upon hot iron ”

“ In a note the author remarks, that on more than one authority steel is said to be less dense after being hardened by quenching than before, which he ascribes, as in glass, to the sudden induration having commenced at the surface. ”

“ Various fibrous vegetable substances have also the property of depolarizing, but have neutral axes in the direction of their fibres. Adipocire from various sources, spermaceti, and soups of all sorts, depolarize in every position. Various kinds of hair, wool, feather, and silk depolarize, but have neutral axes in the direction of their length, and at right angles to it.
Human cuticle, human nail, and various kinds of horn; the cornea of the eye of a man, cow, or fish ”

“ 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 are the phenomena produced, when two surfaces artificially polished in the direction of the supposed plane are applied to each other ; but they are produced when a thin film of sulphate of lime is interposed as a vein between two such portions of Iceland spar, and the surfaces are closely united by a cement of strong refractive power. ”

“ IN this continuation of the author's remarks on Rupert's drops, he observes, that when they are made of colourless glass, and are held before a sheet of white paper, even the unbroken drops exhibit some appearances which he considers as marks of a crystalline texture ”