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OPTICS. “When light falls upon a body of a sombre hue it is partly absorbed; but when it falls upon a white substance, or a polished surface, it is more or less completely reflected. The angle of reflection is equal to the angle of incidence. The reflection of light is variously modified by the forms of the surfaces from whence it arises; as from convex, concave, cylindric, and other mirrors. “Some diaphanous bodies possess the property of dividing the ray of light which traverses them into two points, one of which follows the law of ordinary refraction, and the other a particular law, which was discovered by Huyghens. “Transparent carbonate of lime exerts this action in a high degree. The angle of ordinary refraction always bears a ratio to the angle of incidence: the angle of extraordinary refraction depends upon the direction of the ray with regard to the axis of refraction (a line which coincides with the axis of crystallization in carbonate of lime.) When the ray is directed in a perpendicular or parallel direction to this axis, there is no extraordinary refraction; but when it is inclined to it, the refraction is greater or less, according to the angle of inclination. “Light thus refracted is endued with some particular properties. When it is again made to pass through a rhomboid of double refracting spar, whose axis is parallel to that of the original crystal, it passes on without suffering any division: but if the second rhomboid be turned slowly round while the first remains stationary, each of the pencils begins to separate into two: and when the eighth part of a revolution is completed, they ar
rive at their furthest point of division: when the fourth part of a revolution is effected, the pencil refracted in the ordinary way by the first crystal is wholly refracted in the extraordinary way by the second; and that refracted in the extraordinary way by the first, is ordinarily refracted by the second, The same phenomena occur at every quadrant of the turn. Light which possesses these properties is called flolarised light, and its peculiarities are supposed to depend upon a peculiar relative arrangement of its particles, in which their axes and similar faces are all similarly disposed. “This modification is not conferred solely by refraction. Malus has discovered, that light reflected from various substances at certain determinate angles for each, is endued with the same properties. This angle in glass is 35°.” “Polarised light is affected in a particular manner by reflecting surfaces. When a second reflecting plane is placed parallel to the first, the ray is wholly reflected; but when the new plane is perpendicular to the original one, it is, on the contrary, entirely refracted. The intermediate degrees are characterised by intermediate quantities of absorption and reflection. Polarization may also be conferred by ordinary refraction. Thus, in passing through glass, light is polarized in part; and if we transmit it through a series of parallel glasses, part of the molecules which have escaped the operation of the first are detained by the second, and another portion by the third: so that at last, if the number be sufficient, a completely polarized ray is obtained.
“There is another modification of light which is amongst the recent discoveries of the present day. It is supposed to arise from an oscillation of the particles around their centres of gravity. If a ray of polarized light be made to pass through a thin leaf of mica, or selenite, and then analysed by a rhomboid of double refracting spar, it no longer passes through single, but two images are produced, of different colours, which are complementary to each other, that is to say, which produce white light by their mixture. The ray which falls upon the mica penetrates entire to a small depth, without the axes of its particles experiencing any deviation from their position; but at a certain depth, which is different for the different coloured particles, they begin to oscillate like the balance of a watch. These oscillations are confined to the same limits, but vary in velocity. The violet particles turn more rapidly than the blue, they more rapidly than the green, and so on to the red, which are the slowest of all. From this inequality it happens, that for every thickness of the leaf, different colours are found at the two limits of oscillation; and from hence arise the two differently coloured pencils, which are observed in analysing the transmitted light. “ Various experiments prove, that the light of the sun is composed of particles of different colours which are differently refrangible and reflexible. The separation of ...these particles is termed the dispersion of light, and upon it depends the beautiful Newtonian theory of colours.” The above is an extract from Mr. Brande's account of Beudant's
Cours elementaire et generale des Sciences Physiques. When a ray of light enters a crystal whose primitive form is neither a regular octahedron, or a a cube, it is generally observed to be divided into two bundles or fasciculi unequally refracted. One is termed the ordinary fasciculus which follows the law of refraction discovered by Des Cartes, and which is common to all bodies whether crystallized or not; the other, which is termed the eartraordinary fasciculus, follows a different and more complicated law. Huygens determined this last law, by observations on the double refracting spar, Iceland chrystal, or diaphanous rhomboidal carbonat of lime, La Place combining this fact, with the general princi. ples of mechanics, deduced a general formula for the velocity of the luminous particles of the extraordinary fasciculus. This formula, indicates that the particles of light are separated by a force emanating from the axis of the chrystal, which in the double refracting spar, is repulsive. Malus however, may be considered as the first to whom we owe the modern ideas of the polarization of light, since pursued with much success by Biot, Arrago, and Pouillet in France, and Mr. Porret, and Dr. Brewster in Great Britain. The experiments on the polarization and depolarization of light, (its refraction and diffraction) in its passage through various transparent substances, or coloured fringes, and on the phenomena of its reflection from glass and metallic mirrors, within this twelve month have been very numerous. It is deducible generally from the facts announced, that light in
its passage through diaphanous substances composed of laminae— or subject to internal crystallization —or that by mechanical force, or by heat, may be altered as to the internal structure of their particles—or that may be changed in form, as by bending into a concave and convex surface, a plane piece of glass—have the property of acting upon light in its passage through them, so as in some cases to divide, and in others to reflect the rays of the fasciculi; effects which are modified by the direction in which the rays fall on the polarizing body, perpendicularly or obliquely. These experiments have been varied by the scientific observers above mentioned in a great variety of ways, as to the kind of substances employed, the employ of one or two substances, and the different directions in which they were made to receive the rays of light. The fact first observed by Malus is in conformity with all the later discoveries; viz. that if a pile of glass in parallel plates be placed in the direction of a polarized ray, forming with it an angle of 35° 25', the ray produces no reflected light from any of them; hence he concluded at first, that although the light of an ordinary ray or fasciculus of rays, would have been reflected, yet in the actual case the light passed through this whole series of diaphanous bodies. But having made the incident ray to revolve on its own axis, without changing its place, it was entirely reflected by the successive action of the plates of glass, and was no longer distinguishable at the bottom of the pile; but continuing to revolve it, after it had made the revolution of half a circle, it again passed through the plates of the glass pile. This ex
periment (Malus observes) presents the singular phenomenon, of a body which at sometimes appears diaphanous, and at others opake, while receiving not only the same quantity of light but even the same ray of light under the same inclination. A writer in the Annales de Chimie for March 1816, p. 316 (note), probably M. Arago, observes, that most of the results announced by Dr. Brewster in his Memoir on the depolarization which light suffers, in passing through various substances of the mineral, vegetable, and animal kingdoms, are to be found in a memoir of Malus published in one of the Moniteurs of 1811, and announced in the Analyse des Travaux de la Ire classe de l’Institut pour l’Année 1811. Sir Ey. Home has also been complained of, for neglecting to cite the previous corresponding observations of the continental philosophers. This shows the necessity of early information to the scientific world, concerning the subjects about which men of science are occupied, so that we may be freed from the mortification of exclaiming, pereant qui ante nos nostra dixerunt. Mr. Brande informs us, that Dr. Brewster has lately discovered, that white light may be decomposed into its complementary tints by simple reflection from the separating surfaces of transparent media either solid or fluid, not only that have different, but that have the same refractive and dispersive power. The experiments that led to this discovery was placing a film of oil of cassia, and at other times of oil of cassia diluted by oil of olives, between two prisms or plates of flint glass; the light reflected from the first surface of the fluid film will
be of an uniformly brilliant blue colour, while the transmitted light has a pale straw-yellow tint. The papers are not yet published. The French, or rather the Inventor at Paris, whose name at present I have not discovered, has introduced plano-cylindrical lenses; in which the plane surfaces are in contact, and the axes at right angles. Suppose a cylinder of glass, cut through lengthwise, and the two halves joined by their flat surfaces, but placed across each other at right angles, and then cut down to the size of the lens required. These glasses are advertised thus, Par Brevet d’Invention, conservès a surface de cylindres, Chamblant, ingenieur, opticien, breveté du roi, Rue Basseforte St. Denis No. 26 a Paris. Chamblant is not the inventor, but the contractor. This invention interferes so much with the stock on hand of the opticians, that it is with great difficulty the inventor could get workmen to grind or set his glasses. It is suspected that the influence of the Trade, has extended even to the Institute, whose committee were several years in reporting upon the merit of this invention referred to them; and I know not whether they have reported yet. I see by Thompson's Annals for this year, that they are likely to experience a similar opposition in England. It is pretended that with a larger field of vision they exhibit a perfect image of the object, in no wise distorted, without any of those coloured fringes which Dolland was at so much pains to correct; and that they are in these respects manifestly superior to the common lenses. And they are so. Some of them pave been brought to Philadelphia, but the opticians will not import them, for obvious reasons. They have so much merit,
however, that engravers, watchmakers, mineralogists, those who use spectaeles, and all who require glasses of magnifying powers, will have them when they have once tried them. In telescopes they supersede Dolland’s ingenious method of correcting the aberration of the rays, and no more than an eye glass and an object glass is required. The inventor (not Chamblant) in a programme on the subject, has attempted to show mathematically that glasses of this construction ought from the theory to possess these points of superiority over common lenses: I have not seen the programme, but I have tried the glasses and am satisfied. * There are some observations on the superior distinctness of image afforded by concave over convex and plano-convex glasses, in Dr. Herschell's late account of his telescopic apparatus. Month. Mag. for February 1816, p. 51.
BOTANY AND AGRICUL- TURE.
On the continent of Europe, the method of Jussieu founded chiefly on the presence, number, or absence of the cotyledons of plants, bids fair to supersede the Linnaean classification. In England, I believe Brown, is the only botanist who follows Jussieu; Dr. Smith, Mr. Roscoe, and the other scientific gentlemen engaged in botanical pursuits, still adhere to the Linnaean system; which indeed is so good, and has done science so much service, that it is not likely to be thrown aside even for a better. The followers of Jussieu cannot dispense with citing the Linnaean synonimes, though the natural method of the French botanist possesses many advantages.
In France and Germany, vegetable anatomy and physiology, have of late years made more progress than in England, where Ray, Grew, and Hales, contributed largely indeed to the advancement of this branch of science, but they have not had many followers of equal repute among their own countrymen, though Mr. Knight has contributed many important facts and views. On the continent, Duhamel, La Metherie, Mirbel, Decandolles, Palissot de Beauvois, and many others, were, or still are, labouring in the same vineyard; and botany now seems likely to become a science, instead of a mere system of nomenclature: an observation which may well apply also to the present state of mineralogy. La Metherie, the respectable conductor of the Journal de Physique, a man of very extensive knowledge, and great research, but with an imagination that sometimes overruns his judgment, in his first Number, for January 1816, in giving an account of science for the year 1815, has noticed some conclusions of Palissot de Beauvois, and closed them with a brief view by himself, of the analogies between plants and animals, which I think has interest. Palissot de Beauvois (Jour. de Phys. Jan, 1816, p. 20) has published observations on the arrangement and disposition of leaves, on the pith (moelle) of ligneous vegetables, and on the conversion of cortical layers into wood. He has drawn the following conclusions. 1st. The form of the medullary case, or envelop of the pith (etui medullaire), varies in the ligneous dicotyledons. These variations are subservient to a constant law, and depend on the arrangement either of the branches or the leaves. He
has remarked in this case (etui) five different forms; a. Triangular: as in the laurel rose; where the leaves are verticillated by threes. b. The Tetragon: in trees, such as the linden, wherein the spiral formed by the leaves is composed of four leaves. c. The Pentagon: in trees such as the oak, the chesnut, &c. when the spiral is composed of four leaves. d. The Polygon: in pines, where the leaves are scattered. e. The round or oval: in trees where the leaves are placed opposite to each other. 2dly. The pith (moelle) is absolutely necessary to sustain the life of the vegetable during its youth: but in old trees as in willows deprived of pith, it is supplied by medullary radiating fibres. 3dly. The monocotyledons have no pith like the dicotyledons. Yet Rumphius, Daubenton, &c. remarked in the palm tree and some other plants, a substance analogous to the pith and medullary rays. DuPetit Thouars particularly, informs us of the pandanus, the dracaena, and other monocotyledons, that differed greatly in this respect from others of the same class. The gramens, the bamboos, &c. offer some still more remarkable exceptions in this case. 4thly. The recent woody layers are produced by the liber, and not as Hales supposed by wood precedently formed; just as in animals, the recent layers of bone are the produce of periosteum.”
* M. Mirbel had long insisted that the liber changed into wood: M. Du Petit Thouars, and Mr. Knight, opposed this doctrine strenuously. M. Mirbel has come forward in article 5 of the