Page images
PDF
EPUB

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.

N

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, SN, SM, 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

Fig. 276.

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.

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

[graphic][subsumed]

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

[ocr errors]

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 gingular 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 lawsit 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.

[graphic]

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 a a a a. 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. 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.

It

Fig. 279.

a

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.

With respect to the accessory parts, they consist chiefly of the eyelids, which serve to wipe the face of the eye, and protect it from accidents and dust; the lachrymal apparatus, which serves to wash it with tears, so as to keep it continually brilliant; and the muscles, requisite to direct it upon any point.

Of the nervous part of the eye, so far as its functions are concerned, but little is known. The retina receives the impressions of the light, which are conveyed along the optic nerve to the brain.

Now, as respects the optical action of the eye, it is obviously nothing more than that of a convex lens, with which, indeed, its structure actually corresponds; and as in the focus of such a convex lens objects form images, so by the conjoint action of the cornea and crystalline, the images of the things to which the eye is directed form at the proper focal distance behind—that is, upon the retina. Distinct vision only takes place when the cornea and the lens have such convexities as to bring the images exactly upon the retina.

In early life it sometimes happens that the curvature of these bodies is too great, and the rays converging too rapidly, form their images before they have reached the posterior part of the eye, giving rise to the defect known as short-sightedness-a defect which may be remedied by putting in front of the cornea a concave glass lens of such concavity as just to compensate for the excess of the convexity of the eye.

In old age, on the contrary, the cornea and the lens become somewhat flattened, and they cannot converge the rays soon enough to form images at the proper distance behind. This long-sightedness may be remedied by putting in front of the cornea a convex lens, so as to help it in its action.

Concave or convex lenses, thus used in front of the eyes, constitute spectacles. It is believed that this application was first made by Roger Bacon, and it unquestionably constitutes one of the most noble contributions which science has ever made to man. It has given sight to millions who would otherwise have been blind.*

The image which is formed by a convex lens being inverted as respects its object, so must the images which form at the bottom of the eye. It has, therefore, been a question among optical writers, why we see objects in their natural position, and also why we do not see double, inasmuch as we have two eyes. Various explanations of these facts have been offered, chiefly founded upon optical principles. None, however, appear to have given general satisfaction, and in reality, the true explanation, I believe, will be found not in the optical, but in the nervous part of the visual organs. It is no more remarkable that we see single, having two eyes, than that we hear single, having two ears. It is the simultaneous arrival in the brain that gives rise out of two impressions to one perception, and accordingly, when we disturb the action of one of the eyes by pressing on it, we at once see double. Among the peculiarities of vision it may be mentioned, that for an object

* A defect, and not an uncommon one, with some persons, consists in the eyes refracting the rays of light with different powers in different places. The defect may be detected by making a small pin-hole in a card, which is to be moved from close to the eye to an arm's length, while the gaze is directed toward the sky, or some bright object. If the sight be perfect the hole does not alter its circular form at all; but if the peculiar defect exists, the hole becomes elongated, and ultimately merges into a straight line. M. Airey considers that a spherical cylindrical lens will correct the defect, as it succeeded in his own case.-ED.

to be seen it must be of certain magnitude, and remain on the retina a sufficient length of time; and, for distinct vision, must not be nearer than a certain distance, as eight or ten inches. This distance of distinct vision varies somewhat with different persons. The eye, too, cannot bear too brilliant a light, nor can it distinguish when the rays are too feeble; though it is wonderful to what an extent, in this respect, its powers range. We can read a book by the light of the sun or the moon; yet the one is a quarter of a million times more brilliant than the other. Luminous impressions made on the retina last for a certain space of time, varying from one-third' to one-sixth of a second. For this reason, when a stick, with a spark of fire at the end, is turned rapidly round, it gives rise to an apparent circle of light.

By accidental or physiological colours we mean such as are observed for a short time depicted on surfaces, and then vanishing away. Thus, if a person looks steadfastly at a sheet of paper strongly illuminated by the sun, and then closes his eyes, he will see a black surface corresponding to the paper. So if a red wafer be put on a sheet of paper in the sun, and the eye suddenly turned on a white wall, a green image of the wafer will be seen. Spectral illusions in the same way often arise thus, when we awake in the morning, if our eyes are turned at once to a window brightly illuminated, on shutting them again we shall see a visionary picture of every portion of the window, which after a time fades away.

CHAPTER XLVI.

OF OPTICAL INSTRUMENTS.

The Common Camera Obscura-The Portable Camera-The Single Microscope-The Compound Microscope-Chromatic and Spherical AberrationThe Magic Lantern-The Solar Microscope-The Oxihydrogen Micro

scope.

In this and the next chapter I shall describe the more important optical instruments. These, in their external appearance, and also in their principles, differ very much according to the taste or ideas of the artist. The descriptions here given will be limited to such as are of a simple kind.

THE CAMERA OBSCURA, or dark chamber, originally consisted of nothing more than a double convex lens of a foot or two in focus, fixed in the shutter of a dark room. Opposite the lens, and at its focal distance, a white sheet received the images. These repre

[graphic]

sent whatever is in front of the lens, giving a beautiful picture of the stationary and movable objects in their proper relation of light and shadow, and also in their proper colours.

In point of fact, a lens is not required for if, into a dark chamber, CD, Fig. 280, rays are admitted through a small aperture,

Fig. 280.

L, an inverted image will be formed, on a white screen at the back of the

M

« PreviousContinue »