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Airy.—Mathematical Tracts. 2d edit. 1831. Also, Transactions of the Cambridge Philosophical Society.

Pouillet.Elémens de Physique Expérimentale et de Météorologiė, 2 vols. 1827.

Peclet.Traité Elémentaire de Physique. 2me. ed. 2 vols. 1830.

Quetelet. — Notes to the French Translation of Herschel's Article on Light. Also, Positions de Physique. 1834.

Lloyd, Rev. H.—Report on the Progress and Present State of Physical Optics, in the Report of the Fourth Meeting of the British Association. 1835. Also, Lectures on the Wave Theory of Light. 1841.

Powell, Rev. Professor.—Elementary Treatise on Experimental and Mathematical Optics. 1833. Also, a General and Elementary View of the Undulatory Theory, as applied to the Dispersion of Light. 1841. Likewise, various papers in the Philosophical Magazine.

Rose, Gustav. — Elemente der Krystallographie. 2te. Aufl. 1838. (A French translation of the 1st edition of this work).

Dove.- On the Circular Polarization of Light: translated in Taylor's Scientific Memoirs, vol. i.

Soubeiran.-Journal de Pharmacie. 1842.
Ventzke.-Annals of Chemistry. December, 1842.

The author takes this opportunity of offering his warmest thanks to his friend, Mr. Woodward, for his valuable assistance and advice on many occasions, in the performance of experiments on Polarized Light; as well as for the loan of various pieces of apparatus, contrived and adapted by Mr. Woodward, for the public illustration of the phenomena of Polarized Light.

To Mr. Darker, optician and manufacturer of polarizing apparatus, of Paradise Street, Lambeth, the thanks of the author are also justly due, for his disinterested zeal, skill, and attention in promoting the objects of the author in the preparation of the present course of Lectures.

J. P. 47, FINSBURY SQUARE,

September, 1843.

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LECTURE IV.

4. CIRCULAR POLARIZATION.—General explanation. Conditions neces-

sary for its production. Different methods of effecting it. Fresnel's

method, his rhomb. Airy's method. Dove's method. Quartz, its peculiar

optical properties. Right-handed and left-handed quartz. Plagie-

hedral quartz. Circular Polarization of Fluids ; liquids possessing

this property, others which do not. Description of the apparatus for ob-

serving the circular polarization of liquids, Biot's, Powell's, Ventzke's.

Volatile oils, their power of rotating the planes of polarization ; ap-

plication. Sugar, rotative powers of solutions of different kinds of

saccharine substances. Applications, sugar-refining, fictitious manna,

vinous fermentation, diabetic urine. Dextrine. Properties of circu-

larly polarized light; its reflection, transmission through tourmaline

plates, double refraction, conversion into plane polarized light by

two internal reflections, uniform tints and coloured rings produced

by its transmission through crystalline films, transmission unchanged

through circularly polarizing liquids, action of quartz, production of

spirals, Earnshaw's theoretical deduction, and Powell's verification

of it. Airy's analyzer for circularly polarized light ...................86—106

5. ELLIPTICAL POLARIZATION.—Conditions for its production. Dif-

ferent modes of effecting it. Distinguishing characters of elliptically

polarized light .................................

UU ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........106-108

6. MACLES AND COMPOUND CRYSTALS.-Hemitropes, intersecting crystals,

twin or double crystals, tesselated or composite crystals. Nitre,

Arragonite, Amethyst, Topaz, Sulphate of Potash, Apophyllite,

Analcime, idiocyclophanous crystals of Iceland spar, &c.............108–110

Conclusion.

... 110

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ON

THE POLARIZATION OF LIGHT,

&c. &c. &c.

LECTURE I. With the concurrence of the Council of your Society, I have undertaken to deliver three lectures on the Phenomena of Polarized Light. I have done so, because I believe that their singularity, variety, splendor, and useful applicability will create great and universal interest in the minds of my auditors, whatever be their pursuits, occupations, or acquirements.

I am acquainted with no branch of experimental philosophy capable of presenting such brilliant and gorgeous phenomena, and which are so well adapted for illustration in the lecture-room, as polarized light. In its power of unfolding to our view the intimate structure and constitution of natural bodies, it certainly has no superior, if indeed it have any equal. It furnishes us with characters for recognizing and distinguishing many bodies, and it gives us the means of determining the nature of the changes going on in some of the recondite operations of nature. It is a subject whose phenomena are so complicated and intricate, that it not only admits of, but actually requires, the highest departments of mathematics to elucidate them; and it is, therefore, very properly placed in the very first rank of the physico-mathematical sciences.

But in all societies and associations, the lovers of knowledge are of two kinds, philosophers and utilitarians. The first pursue science for its own sake, the second for its usefulness. With the latter every step they take in the acquirement of knowledge is accompanied with the question “ cui bono?” With such, all scientific researches which have no immediately practical bearing, which, according to their narrow views, cannot be at once shown to be useful, are neglected, perhaps even sneered at. Though with such I profess to hold no community of feeling; yet as I am desirous of combining in these lectures, the utile with the dulce, I think I can venture to hold out to them ample remuneration for the time they may devote to the study of polarized light, by attending these lectures.

If I can show then that this agent furnishes us with a more intimate knowledge of the nature and properties of those substances, by the commerce in which most of the Members of this Society gain their bread ; if I can demonstrate its applicability to the detection of adulteration of foods, drugs, and chemicals; if I can point out its application to the determination of the commercial value of saccharine juices; if I show how it has been applied to determine the nature of the changes which occur in certain chemical and vital processes, in which ordinary che. mical analysis completely fails us; if I prove that it may aid members of my own profession in detecting the existence of certain diseases; and, lastly, if we show the possibility of its use to the mariner in aiding him, under certain circumstances, to avoid shoals and rocks—I trust even the utilitarians will admit that the study of polarized light is both advantageous and profitable, and that the time of this Society has not been unprofitably occupied by these lectures.

These are only a portion of the valuable and practical uses of which polarized light is susceptible. Its phenomena are so intricate, and at present so little understood by the public, that a very large number of persons, who might otherwise perhaps beneficially avail themselves of its services, are ignorant alike of its powers and of its uses. We may, therefore, hope that when it becomes better known it will be found more extensively useful.

Common and polarized light agree in several of their leading properties, and though these lectures are intended to illustrate the peculiarities of polarized light, yet before we can prove what is peculiar to the one, we must be acquainted with the general properties of the other, and thus, I conceive, I must introduce polarized light to your notice, by a preliminary general view of the physical properties of light.

Moreover, the phenomena of polarized light are so numerous, various, and intricate, that the student is very apt to become bewildered with an immense multitude of facts, and to forget, if indeed he ever knew, the conditions which are requisite for the production of each phenomenon. Hence, then, it becomes desirable that we should give him some artificial aid to assist in the conception of facts, and the modes of observing them; as well as to show him how these manifold phenomena are mutually connected and dependent. We require in fact some means of generalization. Such will be found, I think, in the undulatory hypothesis of light.

I propose, therefore, to occupy this lecture with a brief statement and demonstration of the properties of light, and to take a hasty glance at the hypothesis of waves or undulations; so that I trust you will leave this room to-night with some general notions

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