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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 of the intensity, at thrice the distance the intensity, 급 at four times the distance 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 1, 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

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. In 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 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

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Wheatstone's Photometer.

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 Cross.

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

body in different states of density. Thus the apparent crookedness of a stick placed obliquely in water; the difficulty of hitting a body, as a fish, in water, when we take an oblique aim; the deception experienced in estimating the depth of water, except when viewed perpendicularly; and the altered position of a body (as a piece of money) contained in a basin, when viewed obliquely, first when the basin contains no water, and afterwards when water has been put in-these, and many other phenomena, result from the greater refractive powers of water than of air, and the consequent change of direction which the luminous rays suffer when passing from one medium to the other. Again, the tremulous motions of bodies, when viewed through an ascending current of heated air, and by which an excise-officer is said to have, on one occasion, discovered a subterranean still in the Highlands of Scotland, result from the unequal refracting power of air in different states of density.

6. Dispersion.-If a ray of white light be made to traverse a refracting medium, or, in other words, to suffer refraction, it is found to have undergone a remarkable change--it is no longer perfectly white, but more or less coloured. It is assumed, therefore, that white light is made up of coloured lights, and that these, being unequally refrangible, are separated, or, in optical language, are dispersed. In this way, seven colours are obtained, viz. violet, indigo, blue, green, yellow, orange, and red. These are usually procured by a triangular piece of glass, called a prism-the seven colours constituting the prismatic or solar spectrum. This mode of producing colours from white light is called the decomposition, the analysis, or the dispersion of light. If we allow the oxyhydrogen lime-light to pass out of the lantern through a slit, and receive it on a prism, the spectrum may be thrown on the cieling of the lecture-room, or on the screen before us.

Το persons unacquainted with philosophical investigations, few facts seem more astonishing, and even improbable, than that of white light being compounded of differently coloured lights. I shall, therefore, dwell for a few minutes on this topic.

Every one is familiar with the fact, that, by mixture, colours are altered. Thus blue and yellow form green; red and yellow form orange; while blue, with different proportions of red, yields indigo or violet.

You will, therefore, readily believe, that of the seven prismatic colours into which the prism decomposes white light, three only may be primitive, and four compounded.

Primitives.
Red
Yellow

Compounds.
Orange

Blue

Green

Indigo

Violet

If the seven prismatic colours be rudely printed on a circular disk of card, and then be made to rotate rapidly, the union of these colours on the retina gives us an impression of greyish-white.

If we paint the three supposed primitive colours, viz., red, yellow, and blue on a similar disk, and cause this to revolve, we also obtain an impression of greyish white.

These experiments, therefore, favour the notion that the sensation of white light depends on the simultaneous impression of differently-coloured lights on the retina; and, secondly, that three of the prismatic colours being capable of giving the sensation of white light, they probably are the primitive colours, the others being compounds. Hence, white light is called compound or heterogeneous light; while the three colours, red, yellow, and blue, are termed simple or homogeneous lights. Each of these may be termed a monochromatic light. Orange, green, indigo, and violet, on the other hand, are mixed colours. It follows, from this view of the subject, that two colours (one of which must be a mixed colour) may by their union or mixture produce white light. Colours or tints which do this are called complementary.

Complementary Tints.
Red... and Green.
Yellow "Indigo.
Blue...

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Orange.

White and black are also said to be complementary.

I shall now proceed to demonstrate the accuracy of these positions. If I throw two beams of coloured light, one red, the other green, on a screen, we see two circular disks of coloured light, and by making them overlap, they produce white

light. A similar result (that is, the formation of white light) is also produced by the overlapping respectively of disks of indigo and vellow, and of blue and orange. These colours are obtained by a complicated process. The oxyhydrogen lime-light is refracted by the condensers in this lantern then polarized then doubly refracted or depolarized by a thin film of selenite-then refracted by the two powers-then analysed by a double refracting prism.

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By this process, the nature of which will be fully explained hereafter, we have destroyed the yellow and the blue, leaving the red, of one beam-while the red only has been destroyed in the other beam, leaving the yellow and the blue (which by their mixture constitute green). If we then cross the two beams, the red and the green by their mixture form white light.

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