full moon, yet the best computations find it to be only a three hundred thousandth part of the brilliance of the meridian sun. The dullest white cloud of day, if its light could be reserved and it could be suddenly introduced at night by the side of the moon, would outshine it. This may be imagined as we contrast these objects in the day-time.

1. Uranus, the planet most distant from our sun, receives, on an equal portion of its surface, only a two thousandth part of the light which falls on Mercury, the planet nearest to that luminary; or, about a three hundred and fiftieth of that which a similar portion of our own surface receives; but this is equal, nearly, to the light of 900 full moons.

2. When the moon is young, the darker portion of her disk (see B on p. 15) appears like a part of a smaller body; and it is difficult to persuade one's self that the extremities of her bright cusps or horns are really portions of an unbroken outline.

3. It is equally hard to believe, that the dilated appearance of the moon at her rising or setting, is owing to the warping of the judgment whilst we compare her with the terrestrial objects which intervene. If a large sheet of stiff paper (as of music paper) be rolled up, and carefully narrowed, until, on looking through it as a tube, we can exactly take in the circumference of the horizontal moon, it will be found, if tied up securely, to be sufficient only for taking in her circumference when she is at her greatest altitude. Astronomical measurement, indeed, can detect a real increase in her disk when considerably elevated, as she is then nearer to us.

REFLECTION AND ABSORPTION OF LIGHT.

Self-luminous bodies, such as stars, flames of all kinds, and bodies which shine by being heated or rubbed, are those which possess in themselves the property of discharging light. Non-luminous bodies, are those which have not the power of discharging light of themselves, but which throw back the light which falls upon them from self-luminous bodies."

1. "When a lighted candle is brought into a room, the form of the flame is seen by the light which proceeds from the flame itself; but the objects in the room are seen by the light which they receive from the candle, and again throw back, (or reflect,) whilst other objects, on which the light of the candle does not fall, receive light from the white ceiling and walls, and thus become visible.”—Sir D. Brewster's Optics, p. 1, 2.

2. Thus the sun and fixed stars are self-luminous; but the moon and planets owe their appearance alone to the light thrown upon them from the sun, and reflected in all directions from their roughened surfaces.

REFLECTION AND ABSORPTION OF LIGHT.

183

3. The surface of the moon is evidently very mountainous, and the most skilful observers, by means of the best telescopes, have detected remarkable protuberances in Venus. But the mountainous character of the surfaces of all the planets has been inferred from this, that they scatter the sun-beams they receive, and thus appear generally illuminated. A polished ball of silver hung up in a room having black walls and in which there is only one candle, like the convex mirror used in drawing rooms, will reflect, to any one spectator, only one minute figure of the candle flame, and have the rest of its surface dark.

4. If it were not for the reflecting particles abounding in our atmosphere, even the sun would fail to afford us daylight, and the heavens would be black except in that small portion in which his figure appears. The blessing of Twilight, so important especially in some regions of our globe, would be altogether unknown. At the summit of Mont Blanc, in consequence of the deficiency of atmosphere above for reflecting the rays of light, the sky appeared to Mons. Saussure as black as ebony.

N The reflection (or bending back) of light takes place, in some degree, from every surface upon which it falls, or we should not see that surface; but even from those best polished, and consequently reflecting most, all the light that falls is not reflected. Some light always enters the substance, and, if not transmitted through it is lost within it. The general law of the reflection of light is that the course of a beam in returning from any point of a surface at which it has fallen, makes the same angle with the surface that it did in falling upon it.

*

*

1. "One of the most curious properties of bodies in their action upon light, is their power of absorbing it. Even the most transparent bodies in nature, air and water, when in sufficient thickness, are capable of absorbing a great quantity of light. On the summits of the highest mountains, where their light has to pass through a much less extent of air, a much greater number of stars is visible to the eye than in the plains below."-Brewster's Optics,' p. 137.

2." With respect to the propagation of light through water, it has been calculated, that not a tenth part of the incident light can advance five fathoms (thirty feet) downward; and that, even of vertical rays, one half is lost in the first seventeen feet. The depths of the ocean must be always in perpetual darkness."-Dr. Prout's Bridgewater Treatise, p. 256.

*

*

3. "A vertical ray of light, in its passage through the clearest air, has been calculated to lose at least a fifth part of its intensity before it reaches the earth's surface. From this cause, and from the actual condition of the atmosphere, it has been estimated that, under the most favourable circumstances, of 1000 rays emanating from the sun, only 378, at a medium, can penetrate to the surface of the earth at the equator; 228 in the latitude of 45°, (half way between the equa

tor and the pole,) and 110 only at the poles; while in cloudy weather, the several proportions are a great deal less."-Ibid. p. 237.

These statements are to be considered only as approximations to the truth; it is not known whether they are equally applicable to the accompanying rays of heat.

4. Sir John Herschel conjectures, from the rare appearance and want of permanence of the spots seen on Venus, and suspected on Mercury, that we do not see, as in the moon, their real surfaces, but only the intense light which is reflected from their atmospheres, much loaded with clouds, and from the glare of which their surfaces are thus protected.

5. The atmosphere of the moon "was beautifully seen under the most delicate of circumstances, during the annular eclipse of May 1836, when, immediately before the completion of the ring, just before the rims of the two bodies osculated, the light of the sun shot through the moon's atmosphere, mollified into lovely twilight."Professor Nichol's Phenomena of the Solar System, p. 162.

REFRACTION OF LIGHT.

P It may be stated, generally, that when a transparent substance is of uniform density, that is to say, when its component particles are equally and uniformly distributed, the light which enters it passes through it in an undeviating line. When its density varies, the course of the ray varies with it. So, also, if the course of light lie through several substances of different densities, or different refractive powers; as through air and water, or through water and glass, or through glass into water and ultimately into air; in every instance, that inclination of the course of a ray which is occasioned by increased refraction, is towards a perpendicular to the point of the surface at which it enters; as though the force of its movement were gradually hindered, and thus its way perverted, like that of a stone flung forcibly sideways into a pool.

REFRACTION OF LIGHT.

185

This ray, which would proceed in its straight forward path to (0) if the globe were quite empty, will now, on entering the water, be diverted to W (a point nearer than o to the perpendicular Z NP drawn through C.) If the lower half of the globe, instead of being filled with water, were of solid glass, this ray would proceed to G; and if we could have it of diamond, it would proceed to D. Hence, the eye must be placed at W, at G, or at D, to perceive S by means of this beam, according to the "medium" through which the beam comes; and, in looking along W C, or G C, or D C, it would see it more or less elevated by refraction; i. e., as if at W S, or G S, or D S.

The angle SCZ, which the ray makes with the perpendicular Z NCP in falling upon the surface at C, is measured by its sine SN (K, p. 16.) So also, the lessened angle W CP, made by the path of the ray when refracted in its passage onward, is measured by Wn, its sine. The ratio between these two sines SN and Wn, found by dividing one of them by the other, (see R on page 11,) is called the Index of Refraction.

Thus, suppose the length S N to be 60 inches, W n (in water) will be 45 inches; Gn (in glass) will be 40; Dn (in diamond) will be 24 only. Hence, the index of refraction of water is 60 divided by 45 or 1; of glass 60 ÷ 40 or 1; of diamond 60 ÷ 24 or 2.

R

We have learned, (M 3, p. 170,) that the density of any portion of the fluid atmosphere in which we live, and through which we see the stars, &c., depends mainly upon the pressure of the other portion above. Hence, its density is comparatively little in the higher regions, and so its power of refracting rays of light.

Z

D

EL

S

Thus the course of a ray proceeding from a heavenly body to impress our sight, is necessarily bent more and more in its course until it reaches us. If we take 8 d D for portions of our atmosphere of differing height, (and therefore of differing density,) then 1, 2, 3, may be regarded as portions of that ray's course; and the impression with respect to the elevation of S above H (the horizon) will be that it is at EL, or nearer to the zenith than it really is.

1. The displacement by refraction, like that of parallax, (G on p. 18,)

is greater as the object is nearer the horizon; but parallax depresses, whilst refraction elevates, and hence, the difference between these two effects is allowed for in nautical observations.

DECOMPOSITION OF LIGHT BY REFRACTION AND
ABSORPTION.

s The Newtonian theory of light supposes it to consist of "material particles emitted by luminous bodies and moving through space" with a velocity of 192,000 miles per second.

In the Undulatory theory,(a) “an exceedingly thin and elastic medium called ether is supposed to fill all space, and to occupy the intervals between the particles of material bodies, * * * and when any vibrations or undulations are propagated through this ether and reach the retina (b) of the eye, they excite the sensation of light." Dr. Brewster's Optics, Cab. Cyclop.

T All the varied beauties of colour result from what, on either of these hypotheses, is called the decomposition of light. Each beam of common light consists of at least three parts, the red, the yellow, and the blue. It is only when a body reflects to the eye all these parts of light, without disuniting them, that it appears white; or when, on the contrary, it absorbs them all, that it appears black. The blue, and yellow, and red, and the various hues resulting from the combinations of these, are all partial reflections of white light, of which the other portions are either being absorbed or scattered in other directions by the bodies presenting them.

✓ When a beam of light is made to pass through the very oblique sides of a glass prism, (as through the cut glass ornaments of a chandelier,) it is decomposed and dispersed; that is, it issues from it beautifully divided into its constituents, in a lengthened figure which is called the spectrum.

(a) First suggested about the same period as Newton's by Huy. ghens, a Dutch mathematician.

(6) The web of nerves behind the eye.