"A picture of any object is formed on the retina of each eye; but although there may be but one object presented to the two eyes, the pictures formed on the two retinæ are not precisely alike, because the object is not observed from the same point of view.

"If the right hand be held at right angles to, and a few inches from, the face, the back of the hand will be seen when viewed by the right eye only, and the palm of the hand when viewed by the left eye only; hence the images formed on the retina of the two eyes must differ, the one including more of the right side and the other more of the left side of the same solid or projecting object. Again, if we bend a card so as to represent a triangular roof, place it on the table with the gable end towards the eyes, and look at it, first with one eye and then with the other, quickly and alternately opening and closing one of the eyes, the card will appear to move from side to side, because it will be seen by each eye under a different angle of vision. If we look at the card with the left eye only, the whole of the left side of the card will be plainly seen, while the right side will be thrown into shadow. If we next look at the same card with the right eye only, the whole of the right side of the card will be distinctly visible, while the left side will be thrown into shadow; and thus two images of the same object, with differences of outline, light and shade, will be formed-the one on the retina of the right eye, and the other on the retina of the left. These images falling on corresponding parts of the retina convey to the mind the impression of a single object;* while experience having taught us, however unconscious the mind may be of the existence of two different images, that the effect observed is always produced by a body which really stands out or projects, the judgment naturally determines the object to be a projecting body.

"It is experience also that teaches us to judge of distances by the different angles of vision under which an object is observed by the two eyes; for the inclination of the optic axes, when so adjusted that the images may fall on corresponding parts of the retina, and thus convey to the mind the impression of a single object, must be greater or less, according to the distance of the object from the eyes.

"Perfect vision cannot then be obtained without two eyes, as it is by the combined effect of the image produced on the retina of each eye, and the different angles under which objects are observed, that a judgment is formed respecting their solidity and distances.

A man restored to sight by couching cannot tell the form of a body without touching it, until his judgment has been matured by experience, although a perfect image may be formed on the retina of each eye. A man with only one eye cannot readily distinguish the form of a body which he had never previously seen, but quickly and unwittingly moves his head from side to side, so that his one eye may alternately occupy the different positions of a right and a left eye; and, if we approach a candle with one eye shut, and then attempt to snuff it, we shall experience more difficulty than we might have expected, because the usual mode of determining the correct distance is wanting.

"In order, then, to deceive the judgment, so that flat surfaces may represent

That this is the correct theory of single vision with the two eyes is evident. For if, while looking at a single object with both eyes, we make a slight pressure with the finger on one of the eyeballs, we shall immediately perceive two objects; but, on removing the pressure, only one will be again seen.

solid or projecting figures, we must cause the different images of a body, as observed by the two eyes, to be depicted on the respective retinæ, and yet to appear to have emanated from one and the same object. Two pictures are therefore taken from the really projecting or solid body, the one as observed by the right eye only, and the other as seen by the left. These pictures are then placed in the box of the stereoscope, which is furnished with two eyepieces, containing lenses so constructed that the rays proceeding from the respective pictures to the corresponding eye-pieces shall be refracted or bent outwards, at such an angle as each set of rays would have formed had they proceeded from a single picture in the centre of the box to the respective eyes, without the intervention of the lenses; and as it is an axiom in optics that the mind always refers the situation of an object to the direction from which the rays appear to have proceeded when they enter the eyes, both pictures will appear to have emanated from one central object; but as one picture represents the real or projecting object as seen by the right eye, and the other as observed by the left, though appearing by refraction to have proceeded from one and the same object, the effects conveyed to the mind, and the judgment formed thereon, will be precisely the same as if the images were both derived from one solid or projecting body, instead of from two pictures, because all the usual conditions are fulfilled; and consequently the two pictures will appear to be converted into one solid body.

"The necessary pictures for producing these effects, excepting those of geometrical figures, which may be laid down by certain rules, cannot, however, be drawn by the hands of man; for, as Professor Wheatstone has observed, 'It is evidently impossible for the most accurate and accomplished artist to delineate, by the sole aid of his eye, the two projections necessary to form the stereoscopic relief of objects as they exist in nature, with their delicate differences of outline, light, and shade. But what the hand of the artist was unable to accomplish, the chemical action of light, directed by the camera, has enabled us to effect.'

FIG. 78.-Brewster's Refracting Stereoscope.

"Daguerreotype portraits and Talbotype pictures are therefore taken, usually, by two cameras placed towards the object, with a difference of angle equal to the difference of the angle of vision of the two eyes, which is about 18° when the object is eight inches from the eyes; hence, if these be carefully examined and compared with the original projecting objects, they will be found to be faithful representations of the object as seen by each eye respectively."

DIRECTIONS FOR USING THE STEREOSCOPE.

"The objects must be so adjusted in the box, that only one picture may be seen in the centre, care being taken that the pictures are not reversed so as to be seen by the right eye instead of the left, and vice versa.

"The proper position of portraits, buildings, and similar objects cannot be mistaken; but where this is not readily perceived, it should be ascertained, when the object can be marked so as at once to be properly placed.

"The eye-pieces, if allowed to turn, are marked with arrows, to indicate their proper position, these must be placed inwards, and in a right line with each other.

"The eye-pieces in some instances are made to draw out to suit the foci of different persons. But those who use spectacles will generally see best with them on, bringing them forward so as to lie flat on the eye-pieces, which in such cases should not be drawn out.

"Persons, however, with a defective sight in either eye will not be able to perceive the astonishing effects of the arrangement, as two different images will not be perfectly formed on the retina of the respective eyes."

FIG. 79.-Example of the zigzag path of Lightning.

PERSISTENCE OF VISION.

There is a most interesting class of experiments that depend chiefly upon another property or faculty of vision, by which we retain for a certain limited period the images of objects presented before us. It may be premised that the term image refers to that picture which remains upon the eye as long as the object is present; whereas the spectrum, which every one knows is the Latin for spectre, is that lingering impression left upon the eye after the real object has been removed. This property, like binocular vision, may be satisfactorily proved in various ways. Thus, if a broom-stick be thrust into the fire and burnt, so as to obtain a mass of ignited charcoal, and then whirled rapidly round in a circle, a complete circle of light is visible. Now,

it is evident that the hand or stick cannot be in every part of the circle at the same instant of time; the mind is therefore obliged to confess, in tracing the stick through the quarter, half, three-quarter, and whole circle, that of course the impression of the train of light must have remained upon the eyes, or else a single spot of light moving in a circle could only have been visible. A planet, if it moved fast enough, would leave a train of light, indicating, like the burning stick, its particular path or disc. The meteors move with such amazing velocity that their trains of light are extremely vivid, marked, and lengthened out, and show distinctly the direction or path they take. A discharge of natural electricity or lightning would, if it moved slowly, be represented by a ball of fire travelling from one point to another; it is, however, usually represented by a lengthened-out zigzag. (Fig. 79.) It is then called "forked lightning," and every part of its track remaining impressed on the vision, the whole appears as a series of continuous lines of fire, which, although diverted right or left, in a horizontal, perpendicular, or angular direction, pursue their path to the point where the discharge occurs, they are visible as a whole, and called a flash of lightning.

The act of winking the eye is another familiar example of the same truth; the eyelid closes and re-opens so rapidly, for the purpose of lubricating the eyeball, that the object we may be looking at does not become invisible, but remains impressed upon the eye. It has been ascertained that the impression lasts for about the seventh or eighth part of a second, and although sometimes it may last for the third part of a second, it depends, no doubt, upon the amount of sensitiveness belonging to the organ of vision. There are very curious modifications of this property of vision, whereby colours and their complementary tints are impressed upon the eye. Thus, if a red wafer is placed on a sheet of black paper, and well illuminated by a sunbeam or any brilliant light, it will appear again to a spectator looking from the black to a white paper as a green one; the red wafer being the real image, whilst the green one is the spectrum. The experiment may be varied with a yellow wafer on a black ground, which appears violet when the eyes are turned rapidly away to a white surface. On this principle a very entertaining book has been published. The reader, after staring at one of the illustrations, is directed to look up to the ceiling or wall, to observe the spectral effect. Sir D. Brewster explains these curious results, spoken of as accidental colours, by supposing that the eyes, after staring at any particular colour, say a bright red, become so fatigued or partially paralyzed that they cannot receive or appreciate the wave of red light, but as white light is made up of various waves of coloured light, the remaining sets of waves-viz., blue or yellow-can impress the vision by producing the complementary green colour. The late Dr. Golding Bird describes the following mode of demonstrating this fact, giving the merit of the experiment to the late Professor Cowper, who invented so many clever illustrations:

"Cut in a piece of cardboard a series of holes, so that when folded together they will exactly correspond, the whole resembling open lattice-work. Provide some sheets of thin tissue-paper of various colours, selecting those presenting strongly defined tints; place one of them between the folds of the cardboard and hold it up to a vivid light, keeping the eye fixed on the latticework whilst the light penetrates the coloured paper; in a few seconds the white colour of the pasteboard will vanish, and be replaced by a strongly marked tint complementary to that of the paper placed in it. Thus, with

yellow paper the framework will appear violet, with blue it will be orange, and with red it will be green. This illusion is so complete that it always excites surprise in those who see it for the first time."

A little gunpowder placed on a block of wood, with iron filings sprinkled over it, throws up a shower of brilliant sparks of burning particles of iron when fire is applied; and if the experiment is performed in a dark room, and the eyes of those standing near the experiment are closed directly after witnessing the real image of the burning particles of metal, they will see a volume of faint light, sometimes coloured, which remains upon the retina, and forms a spectral image. If the colours of the solar spectrum are painted

on a glass disc, to which rapid motion may be imparted, after being fitted into the oxy-hydrogen lantern, a large disc can be thrown upon the screen, which changes to a greyish white directly it is set in motion. The change of the disc of many colours to a grey is very impressive, and is probably understood better by suggesting that the spectator should look through an aperture made in some opaque screen at the coloured disc; the red, orange, yellow, green, blue, indigo, and violet pass before the aperture with such rapidity that they have not time to impress the retina as single colours, succeeding each other one by one, and they must therefore act collectively on the vision; if collectively, then synthetically; or, in plainer terms, the colours are caused to unite and reconstitute white light, or the nearest approach to it that can be produced by a mechanical contrivance of this nature.

Many years ago the juveniles discovered that by twirling a halfpenny you could see both sides of it; not only the portrait of the reigning monarch, but the usual figure of Britannia. This simple arrangement appears to have been succeeded by a more elegant contrivance, invented by the late Dr.