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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 them 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 chemical 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 desiruble that we should give him some artificial aid to assist in the concept ion 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 of the possible physical causes of common and polarized light, and almost anticipate some of the statements which I shall have to make in the next lecture.

1. GENERAL STATEMENT OF THE PHYSICAL PROPERTIES OF LIGHT.

1. Propagation of Light.—Light emanates from luminous bodies with the enormous rapidity of above 190,000 miles per second. This has been ascertained in two ways; first, by observation of the times at which the eclipses of the satellites of Jupiter are perceived by us at different seasons, according to the part of its orbit which the earth happens to be in; and, secondly, by the phenomenon called the aberration of the fixed stars. The first method gives 192,500—the second, 191,515—miles per second.

2. Variation of Intensity.—The intensity of light decreases as the square of the distance increases. At twice the distance, it has only J of the intensity, at thrice the distance ^ the intensity, at four times the distance -f^ of the intensity, and so on.

The reason of this is, that being highly expansile, it illuminates four times the space at twice the distance, nine times at thrice, and sixteen times at four times the distance; hence, its intensity must be inversely as the square of the distance.

The law is aptly illustrated by a quadrangular pyramid of wood, divided horizontally at equal distances, into four parts or segments of equal height. The upper segment has a square base, whose area we shall call 1. The second segment has also a square base, but its area is 4. The area of the square base of the third segment is 9, and that of the lowest or fourth segment, 16. Here the distances of the bases of the segments from the apex of the pyramid, are as 1,2, 3, 4, while the areas of these bases are as I, 4, 9, 16.

The readiest demonstration of the law for the lecture-room, is the following:—Let the light from a lantern pass out through a square aperture, and be received on a semi-transparent screen, on which square spaces are marked. Notice at what distance the beam of light illuminates one of these squares. At double the distance, it will illuminate 4, at treble 9, at quadruple 16 squares.

In Photometry, we avail ourselves of this law. If two luminous bodies, at unequal distances, produce the same amount of illumination, the relative quantities of light evolved by these bodies, are as the squares of the distances. Thus, if a lamp, at four feet distance, give as much light as a candle at one foot, the lamp actually evolves 16 times as much light as the candle. Count Rumford's photometrical process of observing at what distances two lights gave two shadows of equal intensities, as well as the photometers of the late Mr. Ritchie and of Professor Wheatstone, are on this principle. But all these modes of measuring light

B2

are objectionable, since they are based on the imperfect and

varying judgment of the eye.

Professor Wheatstone's recently-constructed photometer is a

very ingenious contrivance. It is a cylindrical box, of about

two inches diameter, and one inch in depth, and which contains a system of two wheels and pinions. On the face of the box, and near to its external border, is a circle of cogs. la

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'the centre of the face is an axis, to which is attached an horizontal arm, carrying a toothed wheel or disk, the teeth of which fit into the cogs of the outer circle. This wheel has a double wheatstone's Photometer. motion, it rotates on its own

axis, and also revolves within the cogged circle. To this disk is attached a small, hollow, glass bead, silvered internally, and which moves with great rapidity backwards and forwards across the face of the cylinder. The motion is communicated by turning the handle on the opposite face of the box. If this photometer be placed between two lights, and the bead put in rapid motion, we observe two parallel luminous lines, about the -jjq- of an inch apart. By adjusting the relative distances of the two lights from the photometer, so that the brightness of the luminous lines may be equalized as determined by the eye, and then squaring the distances, their comparative intensity may be ascertained*.

3. Transparency and Opacity.—Some bodies allow light to penetrate them, as air, water, glass, crystal, &c. These are called transparent bodies. Others, however, refuse to give passage to light, as the metals. The latter are termed opake bodies. But some substances, which in the mass are opake, become transparent when reduced to thin films. Gold is an instance of this: in the lump it is opake, but as gold leaf it allows light to traverse it.

4. Reflection.—When a beam of light falls on a smoothpolished surface, a portion of it is reflected. The incident and the reflected ray make each the same angle with the reflecting surface, hence the law of reflection is, that the angles of incidence and reflexion are equal. This law holds good under all circumstances, whether the reflector be plain or curved.

A polished metallic plate as a speculum is a good reflector. Glass, being transparent, reflects both from its anterior and

• This instrument is made by Messrs. Watkins and Hill, of Charing Crow.

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posterior surface. Hence in some optical experiments, where it is desirable to avoid the confusion from a double reflexion, the posterior surface of the glass is either ground, or blackened by means of soot, candle-smoke, or size and lamp-black. This proceeding is especially desirable in experiments on polarized light. Silvered glass, that is, glass covered on the posterior surface by an amalgam of tin, as the common looking-glass, is not adapted for accurate optical experiments, on account of the reflection from the metal as well as from the glass

5. Refraction.—When a ray of light passes obliquely out of one medium into another of a different density or combustibility, it changes its direction, or is bent out of its course; in optical language it is refracted. If the second medium be denser, or more combustible than the first, the refraction is towards the perpendicular; but if the density or combustibility of the second medium is less than that of the first, the refraction is from the perpendicular.

If the ray fall perpendicularly on the refracting surface, it suffers no change in its direction, in other words, it undergoes no refraction.

In most optical instruments in which refracting media are required, glass is employed, as in the camera obscura, astronomical and terrestrial telescopes, microscopes, magic lanterns, common spectacles, eye-glasses, &c. The oxyhydrogen apparatus, which I shall use in these lectures for illustrating the phenomena of polarized light, serves, when deprived of its polarizing part, for use as a microscope {oxyhydrogen or gas microscope) the images of the objects being thrown on a screen. Used in this form, it is simply a refracting.instrument. Its structure I shall hereafter explain. Quartz or rock crystal is used, under the name of Brazil pebble, as a refracting medium for spectacles, on account of its greater hardness, and its being less liable to scratch. The diamond and other precious gems have been occasionally used for microscopic lenses. Jewellers employ a glass globe filled with water, to concentrate the rays from the lamp which they use to work by. The water is generally coloured pale blue, to counteract the reddish yellow tint of the artificial light. Amber, when cut and polished, is sometimes used for spectacles. When the object is to concentrate rays of light, and to exclude those of heat, lenses of alum or sulphate of copper may be employed.

I have already stated, that the law of reflection, as regards the direction of the reflected ray, is the same for all reflecting media. But the law of refraction is very different, each refracting medium having its own peculiar action on light.

A variety of curious and well-known phenomena result from the unequal refracting powers of different bodies, or of the same

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