which serves as a screen on which the bow is depicted. When two arches are visible, the inner one is the more brilliant, and the order of its colours is the same in which they appear in the prismatic spectrum-the red fringing its outer boundary, and the violet being within. This is called the primary bow. The secondary bow, which is the outer one, is fainter, and the colours are in the inverted order. When the sun's altitude above the horizon exceeds 42° the inner bow is not seen, and when it is more than 54° the outer is invisible. If the sun is in the horizon, both bows are semicircles, and according as his altitude is greater, a less and less portion of the semicircle is visible; but from the top of a mountain bows that are larger than a semicircle may be seen.

a

BLUE
CREEN

RED

These prismatic colours arise from reflection and refraction of light by the drops of rain, which are of a spherical figure. In the primary bow there is one reflection and two refractions; in the secondary there are two reflections and two refractions. Thus, let S, Fig. 272, be a ray of light incident on a rain drop, a. On account of its obliquity to the surface of the drop, it will be refracted into a new path, and at the back of the drop it will undergo reflection, and returning to the anterior face and escaping, it will be again refracted, giving rise to violet and red, and the intermediate prismatic colours between, constituting a complete spectrum; and as the drops of rain are innumerable, the observer will see innumerable spectra arranged together so as to form a circular are.

Fig. 273.

Fig. 272.

The secondary rainbow arises from two refractions and two reflections of the rays. Thus, let the ray, S, Fig. 273, enter at the bottom of the drop; it passes in the direction towards I' after having undergone refraction at the front; from I' it moves to

I", where it is a second time reflected, and then emerges in front, undergoing refraction and dispersion again. For the same reason as in the other case, prismatic spectra are seen arranged together in a circular arc, and form a bow.

In Fig. 274, let O be the spectator, and O P a line drawn from his eye to the centre of the bows. The rays of the sun, S S, falling on the drops, A B C, will produce the inner bow, and falling on D E F, the outer bow, the former by one, and the latter by two reflections. The drop A reflects the red, B the yellow, and C the blue rays to the eye; and in the case of the outer bow, F the red, E the yellow, and D the blue. And as the colour perceived is entirely dependent on the angle under which

-S

Fig. 274.

the ray enters the eye, as in the case of the interior bow, the blue entering at the angle CO P, the yellow at the larger angle B O P, and the red and the largest A O P, we see the cause why the bows are circular ares; for out of the innumerable drops of rain which compose the shower, those only can reflect to the eye a red colour which make the same angle, A O P, that A does with the line O P, and these must necessarily be arranged in a circle of which the centre is P. And the same reasoning applies for the yellow, the blue, or any other ray as well as the red, and also for the outer as well as for the inner bow.

Another interesting natural phenomenon connected with the refraction of light is what is called "astronomical refraction," arising from the action of the atmosphere on the rays of light. It is this which so powerfully disturbs the positions of the heavenly bodies, making them appear higher above the horizon than they really are, and changes the circular form of the sun and moon to an oval shape. It also aids in giving rise to the twilight.

R

Fig. 275.

Let O, Fig. 275, be the position of an observer on the earth, Z will be his zenith, and let R be any star, the rays from which come, of course, in straight lines, such as RE. Now, when such a ray impinges on the atmosphere at s, it is refracted, and deviates from its rectilinear course. At first this refraction is feeble; but the atmosphere continually increases in density as we descend in it, and therefore the deviation of the ray from its original path, R E, becomes continually greater. It follows a curvilinear line, and finally enters the eye of the observer at 0. This may perhaps be more clearly understood by supposing the concentric circles a a, b b, c c, represented in the figure, to stand for concentric shells of air of the same density, the ray at its entry on the first becomes refracted, and pursues a new course to the second. Here the same thing again takes place, and so with the third and other ones successively. But these abrupt changes do not occur in the atmosphere, which does not change its density from stratum to stratum abruptly, but gradually and continually. The resulting path of the ray is, therefore, not a broken line, but a continuous curve. Now, it is a law of vision that the mind judges of the position of an object as being in the direction in which the ray by which it is seen enters the eye. Consequently the star, R, which emits the ray we have under consideration, will be seen in the direction Or-that being the direction in which the ray entered the eye—and therefore the effect of astronomical refraction is to elevate a star or other object above the horizon to a higher apparent position than that which it actually occupies. Astronomical refraction is greater according as the object is nearer the horizon, becoming less as the altitude increases, and ceasing in the zenith. An object seen in the zenith is, therefore, in its true position.

On these principles the figure of the sun and moon, when in the horizon, changes to an oval shape; for the lower edge being more acted upon than the upper, is therefore relatively lifted up, and those objects made less in their vertical dimensions than in their horizontal. Even when an object is below the horizon, it may be so much elevated as to be brought into view; for just in the same way that a star, R, is elevated to r, so may one beneath

the horizon be elevated even to a greater extent, because refraction increases as we descend to the horizon. Stars, therefore, are visible before they have actually risen, and continue in sight after they have actually set. They are thus lifted out of their true position when in the horizon about thirty-three minutes. In the books on astronomy, tables are given which represent the amount of refraction for any altitude.

What has been here said in relation to a star holds also for the sun, which therefore is made apparently to rise sooner and set later than what is the case in reality. From this arises the important result that the day is prolonged. In temperate climates this lengthening of the day extends only to a few minutes; in the polar regions the day is made longer by a month. And it is for this cause, too, that the morning does not suddenly break just at the moment the sun appears in the horizon, and the night set in the instant he sinks; but the light gradually fades away as a twilight, the rays being bent from their path, and the scattering ones which fall on the top of the atmosphere brought in curved directions down to the lower parts.

M

The phenomenon of twilight is not, however, wholly due to refraction. The reflecting action of the particles of the air is also greatly concerned in producing it. The manner in which this takes place is shown in Fig. 276, where A B C D represents the earth, TRP the atmosphere, and S O, S 'N, S'' M, rays of the sun passing through it. To an observer at the point A, the sun, at S'', is just set; but the whole hemisphere above him, P R T, being his sky, reflects the rays which are still falling upon it, and gives him twilight. To an observer at B the sun has been set for some time, and he is in the earth's shadow; but that part of his sky which is included between P Q R x is still receiving sun-rays, and reflecting them to him. To an observer at C the illuminated portion of the sky has decreased to P Q z. His twilight, therefore, has nearly gone. To an observer at D, whose horizon is bounded by the line D P, the sky is entirely dark, no rays from the sun falling on it. It is therefore night.

Fig. 276.

The action of the atmosphere sometimes gives rise to curious spectral appearances -such as inverted images, looming, and the mirage. The latter, which often occurs on hot sandy plains, was frequently seen by the French during their expedition to Egypt, giving rise to a deceptive appearance of great lakes of water resting on the sands. It appears to be due to the partial rarefaction of the lower strata of air through the heat of the surface on which they rest, so that rays of light are made to pass in a curvilinear path, and enter the eye. In the same

Fig. 277.

way at sea, inverted images of ships floating in the air are often discovered.

Thus, on the first of August, 1798, Dr. Vince observed at Ramsgate a ship which appeared as at A, Fig. 277, the topmast being the only part of it seen above the horizon. An inverted image of it was seen at B,`immediately above the real ship at A, and an erect image at C, both of them being complete and well defined. The sea was distinctly seen between them, as at V W. As the ship rose to the horizon, the image, C, gradually disappeared; and, while this was going on, the image, B, descended, but the mainmast of B did not meet the mainmast of A. The two images, B C, were perfectly visible when the whole ship was actually below the horizon."

These singular appearances, which have often given rise to superstitious legends, may be imitated artificially. Thus, if we take a long mass of hot iron, and, looking along the upper surface of it at an object not too distant, we shall see not only the object itself, but also an inverted image of it below; the second image being caused by the refraction of the rays of light passing through the stratum of hot air, as is the case of the mirage.

The trembling which distant objects exhibit, more especially when they are seen across a heated surface, is, in like manner, due to unusual and irregular refraction taking place in the air.

CHAPTER XLV.

THE ORGAN OF VISION.

The Three Parts of the Eye-Description of the Eye of Man-Uses of the Accessory Apparatus-Optical Action of the Eye-Short and LongSightedness-Spectacles-Erect and Double Vision-Peculiarities of Vision -Physiological Colours.

ALMOST all animals possess some mechanism by which they are rendered sensible of the presence of light. In some of the lower orders, perhaps, nothing more than a diffused sensibility exists, without there being any special organ adapted for the purpose. Thus many animalcules are seen to collect, on that side of the liquid in which they live, where the sun is shining, and others avoid the light. But in all the higher tribes of life there is a special mechanism, which depends for its action on optical laws— it is the eye.

This organ essentially consists of three different parts—an optical portion, which is the eye, strictly speaking; a nervous portion, which transmits the impressions gathered by the former to the brain; and an accessory portion, which has the duty of keeping the eye in a proper working state, and defending it from injury.

In man the eyeball is nearly of a spherical figure, being about an inch in diameter. As seen in front, between the two eyelids, dc, Fig. 278, it exhibits a white portion of a porcelain-like aspect, aa; a coloured circular part, bb, which continually changes in width, called the iris; and a central black portion, which is the pupil.

Fig. 278.

When it is removed from the orbit or socket in which it is placed, and dissected, the eye is found to consist of several coats. The white portion, seen anteriorly at a a, extends all round. It is very tough and resisting, and by its mechanical qualities

serves to support the more delicate parts within, and also to give insertion for the attachment of certain muscles which roll the eyeball, and direct it to any object. This coat passes under the name of the sclerotic. It is represented in Fig. 279, at aaaa. In its front there is a circular aperture, into which a transparent portion, bb, resembling in shape a watch-glass, is inserted. This is called the cornea. It projects somewhat beyond the general curve of the sclerotic, as seen at bb in the figure, and with the sclerotic completes the outer coat of the eye.

Fig. 279.

Its

The interior surface of the sclerotic is lined with a coat which seems to be almost entirely made up of blood-vessels, little arteries and veins, which, by their internetting, cross one another in every possible direction. It is called the choroid coat: it extends, like the sclerotic, as far as the cornea. interior surface is thickly covered with a slimy pigment of a black colour, hence called pigmentum nigrum. Over this is laid a very delicate serous sheet, which passes under the name of Jacob's membrane, and the optic nerve, 00, coming from the brain, perforates the sclerotic and choroid coats, and spreads itself out on the interior surface, as the retina, rrrr. The optic nerves of the opposite eyes decussate one another on their passage to the brain.

These, therefore, are the coats of which the eye is composed. Let us now examine its internal structure. Behind the cornea, b b, there is suspended a circular diaphragm, ef, black behind, and of different colours in different individuals in front. This is the iris. Its colour is, in some measure, connected with the colour of the hair. The central opening in it, d, is the pupil, and immediately behind the pupil, suspended by the ciliary processes, gg, is the crystalline lens, c c-a double convex lens. All the space between the anterior of the lens and the cornea is filled with a watery fluid, which is the aqueous humour; that portion which is in front of the iris is called the anterior chamber, and that behind it the posterior. The rest of the space of the eye, bounded by the crystalline lens in front, and the retina all round, is filled with the vitreous humour, V V.