retina upon which it falls, that is, the lower | distinctly when we look away from it, that is, part of the image is seen above the middle when we look at a point distant from it part of the object, and as this is true of several degrees. This singular affection of every point of the image, the object will the retina will be better understood from appear erect from its inverted image. the following experiment :-At the distance The best way of studying the phenomena of ten or twelve feet, look steadily at one of of vision, is to consider objects as made up two candles placed at the distance of about of points like a stippled engraving. Every a foot from each other. In a short time. point will have its image on the retina. the candle not looked at, but seen indirectly, The points in the upper part of the object will increase in size, and will be surrounded will have their image on the lower part of with a bright ring of yellow light, the light the retina, and vice versa, and those points of the candle itself having a pale blue colour. will, by the law of visible direction, be seen If, in the same manner, two objects not forming the upper part of the object. The very luminous, such as two pieces of paper, direction, therefore, of a line drawn from are placed upon a darkish ground, the one any point of an object through the centre of not looked at directly will vanish and reapthe pupil, would be accurately the direction pear-a fertile source of illusion when faintof the sensation, if the centre of the pupil ly illuminated bodies are seen in a dark were the centre of curvature of the retina, night. When the light of an object is exor, what is the same thing, all the points of tremely faint, it will disappear and reappear the object would be seen in their true place. in irregular succession, even when the eye From this approximate coincidence of the direction of the issuing ray, and the line in which the sensation is returned, objects seem to be fixed* when the eye ranges over any object.

is turned fully upon it, or when it is seen directly by the most sensitive part of the retina. The eye is thrown into a state of painful agitation, and we attempt in vain to obtain sustained vision of the object.

Although, on looking at any object, we We have already referred to the foramen obtain distinct vision of it, yet we do not centrale as the spot where vision is most see every part of it at the same instant distinct; but this is not its only optical proequally distinct. When we wish to see perty. The writer of this article found that any point of it, for the purpose of examining it could be rendered visible, and its diameter that point, every other part of it is indistinct. measured in the following manner :-If when The reason of this is, that we direct the axis the eye has been for some time closed, and, of the eye to the point seen distinctly; and as it were, refreshed by protection from the image of that point is formed upon the light, we direct it to a faint white surface, foramen centrale, or central point of the re- such as that of a sheet of paper iluminated tina already mentioned, while the images of by a wax candle at the distance of ten or all other points are formed upon points of twelve feet, there will be seen on the the retina more or less distant from the paper a dark brown or reddish circular spot, foramen. This indistinctness does not arise shading off into the light of the paper. It from any want of focal adjustment, but it is quickly disappears, and may be renewed by a property of the retina, arising, perhaps, again closing the eye for a few minutes. from the membrane being less pulpy, or This spot is, therefore, in the normal conhaving a finer surface at the foramen, or dition of the eye, less sensitive to light than elsewhere; or if the foramen is really an the other parts; that is, it takes longer time aperture, and the choroid coat behind it the to receive the impression of light from the seat of vision, the rays which fall upon the white paper. If the sensibility of the retina choroid at other parts must pass through has been previously reduced by a long exthe retina, and thus be made less distinct. posure to light, or by an exposure to much But though the retina gives less distinct light, the circular spot is white, shading off vision on the parts of it away from the fora- into the light of the paper. In this abnormal men, it is much more sensitive to light in state of the retina, the foramen is more those parts; and hence it is remarkable quickly affected by light than the rest of the that, when we wish to see an object hardly retina. Hence it follows that, when the visible when we look at it directly, such, general retina is in the best state to receive for example, as one of the satellites of luminous impressions, it receives them more Saturn, or a very faint star, we see it most quickly than the foramen part of it, if it is

not an opening; and that when the general retina is fatigued, or less capable of receiv* There is a slight motion arising from the refrac-ing luminous impressions, it receives them

tion of the different humours. When we look

through spectacles the motion is very great, owing

to the refraction of the lenses.

more slowly than the foramen portion. The angular diameter of the circular spot is about

41°, which corresponds with a foramen about of the finger on the side next the nose, there the thirty-fifth part of an inch in diameter, will be produced "a circle of colours like as it has been found to be by anatomists. those in the feather of a peacock's tail." He These experiments are best made in the observes also," that if the eye and the finger morning when the eye has been long pro- remain quiet, these colours vanish in a second tected from the action of light, and in the of time; but if the finger be moved with a evening when it has been most fatigued. quivering motion, they appear again." In Another part of the retina, much larger observing the effects of pressure, we have than the foramen, is wholly insensible to light found that a gentle pressure on the retina of ordinary intensity, and consequently all ob- produces a circular spot of light. By injects disappear when their images fall upon creasing the pressure this spot becomes dark, that part of it. This, however, is true only and is surrounded with a white ring of light, when we look with one eye, for in binocular shading off into darkness. When, in total vision the image in the other eye does not darkness, the retina is subjected to pressure, fall upon this insensible spot. The portion it gives out light; when it is exposed to of the retina thus insensible to ordinary light, compression increases its sensibility light, is at the entrance of the optic nerve, to light; when it is dilated, under exposure It is about the eighth of an inch in diameter, to light, it becomes insensible to luminous and is about 13° from the foramen on the impressions. side next the nose. In order to observe This property of becoming luminous by this curious phenomenon, place two wafers compression shews itself on many occasions. at the distance of three or four inches from A sudden blow on the head or on the eye each other, and, shutting the left eye, look produces a bright flash of light. In sneezing, at the left-hand wafer with the right eye, and and in blowing air violently through the when its distance from the wafer is about nostrils, two patches of light appear above twelve inches, the right-hand wafer will the axis of each eye, and in front of it, while totally disappear, the spot which it occupied other two luminous spots united into one. being of the same colour as the ground upon appear about the point of the nose when the which the wafers are laid. If, when the eyes are directed to it. In turning the eyewafer is invisible, we open the left eye, it balls quickly by the action of their own will reappear; or, if we alter the distance muscles, the retina is pulled or pressed at of the eye, one side of the wafer will come the place of their attachment to the sclerotic into view the innermost side when we in- coat, and a semicircle of light is distinctly crease the distance, and the outermost side seen opposite each eye, and towards the when we diminish it. The same results will nose. These semicircles, in certain states be obtained if we shut the right eye, and of the retina, are enlarged, and are somelook at the right-hand wafer with the left times expanded into complete circles of eye. In this case the left-hand wafer will light. In certain states of the stomach, acdisappear. But though the base of the optic nerve, or the portion of the retina which it forms, is insensible to the light which falls directly upon it, it is susceptible of receiving luminous impressions from the parts which surround it. If the wafers are laid upon a ground of any colour, the spot on which the wafer has disappeared will have the same colour as the ground on which it lies. But though light of ordinary intensity fails to make an impression on this part of the retina, yet when candles are substituted for the wafers the candle does not wholly disappear, but leaves a sort of faint nebulous light, which has no resemblance to the object from which it proceeds.

companied by headache, a faint blue light floats before the eyes in total darkness, passing across the field of view, and sometimes becoming green, yellow, and red.

In rubbing the eyes, specks or points of light frequently appear, arising either from the pressure being felt on some parts more than others, or from those parts being more subject, from their nature, to the emission of light. That this last cause is the more probable one, may be inferred from a very curious phenomenon observed by the writer of this article. If when the eye has been for some time exposed to the light of a gas or any other flame, we suddenly extinguish it, there will be seen for an instant a great When we consider that the sensation of number, from fifteen to twenty, bright points light is produced by a material impression on the retina, it might have been expected that luminous effects would be produced by pressure made upon the eyeball, and communicated to the retina. Sir Isaac Newton, accordingly, observed, that when we press the eyeball outwards, by applying the point

of light like stars, arranged in a circle, the diameter of which subtends an angle of from 70° to 90°. In this case there is no pressure; and, therefore, the parts of the retina which emit these lights must have the property of retaining luminous impressions longer than the other parts of the retina.

We see the

exact point where a near object strikes a more distant one, a thing which we cannot do with both eyes directed to it. When we see the near object distinctly, the more distant one is doubled; so that in shooting with a rifle or a musket, we cannot use both eyes. Some persons have the faculty of shooting with both eyes open, but when they do this they do not observe, or rather they pay no attention to, the second image of the muzzle of their fowling-piece. In monocular vision, when two objects are brought into the same line, they are not seen with the same distinctness; but this is easily remedied by looking at them through a small aperture, which will show them equally dis

It is probable that the points of the retina, | see the direction in which any object or possessing this property, are those at its point of an object is situated, much more margin, where it is in contact with the points distinctly than with two eyes. where the cells of the vitreous humour have their origin behind the crystalline lens. The subject of binocular vision has recently acquired much interest from its connexion with the stereoscope. That one eye is sufficient for the general purposes of life, is evident from the fact that many persons have spent a large portion of their life before they discovered that they were blind in one eye, and also that those who have lost the sight of one eye by accident or disease, can perform with the other almost all the operations which had been performed by both. Two eyes, however, were required to give symmetry to the human form, and no part of the animal mechanism is more interesting than the contrivance by which the two work tinct. But though we cannot estimate disharmoniously together, and give new powers of observation and inquiry. In vision with one eye the extent of the field of view is about 150°, while, with two eyes, it is about 200o, but this depends upon the position of the eyeball within its socket of bone, and upon the form and size of the cheek bones and brow, An eye much sunk in the head, has a much narrower field than 150°, while projecting eyes have a greater field than 200o. The left eye sees a larger field on the left than the right eye, and the right eye a larger field on the right than the left eye, so that each forms a dissimilar picture of the external world, just as they do in viewing solid objects or scenes in nature.

It was long supposed that with two eyes we saw more brightly than with one, or that the luminosity of objects was doubled. This, however, is a great mistake. Dr. Jurin has proved by experiments which we have carefully repeated and found correct, that the brightness of objects seen with two eyes, is only one-thirteenth part greater than when they are seen with one. We are disposed to modify this result, and to maintain that in the normal condition of the eyes, the brightness of objects in monocular and binocular vision is exactly the same. In examining the state of the pupil, and measuring its diameter in these two states of vision, we find it increased in such a degree as to admit as much light when one eye is shut as when both of them are open, so that in as far as the mere brightness of objects is concerned, the loss of one eye is no disadvantage.

tances with any accuracy by one eye, there are various means, called the criteria of distance, by which we learn to form a pretty correct estimate of distances, but particularly great distances. By these criteria, which are five in number, we are enabled in monocular vision to appreciate approximately the distance of objects.

1. The interposition of numerous objects between the eye and the object whose distance we are appreciating. A distance at sea appears much shorter than the same distance on land, marked with trees, and other objects; and for the same reason, the sun and moon appear more distant when rising or setting on the horizon of a flat country, than when in the zenith, or at great altitudes.

2. The variation in the apparent magnitude of known objects, such as man, animals, trees, doors and windows of houses. If one of two men, placed at different distances from us, appears only half the size of the other, we cannot be far wrong in believing that the smallest in appearance is at twice the distance of the other. It is possible that the one may be a dwarf, and the other of gigantic stature, in which case our judgment would be erroneous, but even in this case other criteria might enable us to correct it.

3. The degree of vivacity in the colours. and tints of objects.

4. The degree of distinctness in the outline and minute parts of objects.

5. To these criteria we may add the sen

While two eyes are necessary for the pur-sation of muscular action, or rather effort, pose of symmetry in the human face, they by which we close the pupil in accommodawere required for other important purposes. ting the eye to near distances, and produce They enable us to see solid objects in a high- the accommodation. er relief, and all distances in nature more perfectly than one eye. With one eye we

With all these means of estimating distances, it is only by binocular vision, that

we have the power of seeing distance within | dle, and the remote distances in the landa limited range.

scape. Hence, when we shut one eye, we have not the power of discovering that the picture is on a plane surface, and all its parts equidistant from us, and consequently the art with which the artist gives relief to the painting by light and shadow, or by the dif ferent magnitude of objects of known size, or by indistinctness of outline, and the other criteria of distance, exercises its whole effect in deceiving us into the belief that the pic ture, portrait, or statue is in relief.

In binocular vision short distances are seen directly by the convergency of the optic axes to the point observed. If the object is very near, it is very difficult to converge them without a great strain upon the eye. We feel this strain to be painful, and when we remove the object to greater distances in succession, the painful feeling is diminished. This uneasiness arises both from the great muscular action necessary to bring the axes of the eyeballs to converge upon an object This influence over our judgment is finely near us, and from the contraction of the pu- shown when we view with one eye photogra pil and the simultaneous action of the eye- phic pictures either of persons, landscapes, brows. When the object is withdrawn the sculpture, or machinery. After a little pracoptic axes open, the pupils expand, and the tice the illusion is perfect, and is aided by the eyebrows rise. Distance is therefore really correct geometrical perspective and chiaro seen with two eyes, and it may be proved, obscuro of the daguerreotype and the talboin opposition to the conclusions of Dr. Berke- type. To this species of relief we may give ley and many other metaphysicians, that the name of monocular, which is always indistance, whether represented by a mathe- ferior to the binocular relief in which we see matical or a physical line is visible in the original, or which is produced in the monocular as well as binocular vision. stereoscope. The relation of these three But though relief, and distance as its rep-kinds of relief, when we look at a plane picresentative, is best seen by two eyes, yet ture, namely, ocular with two eyes, monocuvision with one eye is in the following re-lar with one, and binocular when we see spects superior to vision with two.

1. When we look at oil paintings, paintings on porcelain or any other in which the surface is covered with a varnish, or have a gloss of any kind, the varnish or gloss reflects to each eye the light which falls upon it, from objects in various parts of the room, and consequently renders the picture indistinct. But when we close one eye, we shut out the quantity of light which entered that eye as reflected from a different part of the room, and we consequently render the picture more distinct.*

the original solid or landscape, or its two pictures combined in the stereoscope, may be thus observed. Look at any one of the binocular pictures with both eyes, and they have very little relief. Look at them with one eye either in the stereoscope or out of it, and the relief is increased. Look at them when combined in the stereoscope, and the relief is perfect, and an accurate representation of the original solid or landscape, provided the binocular pictures have been taken at the proper angle.*

3. Monocular vision is superior to binocular vision, because it very frequently happens that the one eye is less perfect than the other, and occasionally that the one is of a different focal length from the other, that is, the two eyes see objects most distinctly at different distances. In the first of these cases, the imperfect image in one eye is so blended or united with the perfect image in

2. A painting, picture, or photograph, seen with one eye, is seen more perfectly from another cause. In these representations upon a plane surface every part of the surface is nearly equidistant from us, and when we view them with two eyes by the convergence of the optical axes upon them, it suffers no change, the muscles of the eyeball are not strained, nor the pupil alternately contracted and expanded in seeing objects at * A large number of the binocular pictures now different distances, as is the case when we executed are not taken as if they were seen by two look at a living man, a statue, or a landscape, human eyes, but by eyes, five, ten fifteen, twenty inthe eyes being now converged in rapid suc- ches, and even many feet apart! Such pictures are cession upon the nose, eyes, and false representations of nature, and indicate the greatears, or upon the objects in the foreground, the mid-est ignorance, and if they are not ignorant, the greatest dishonesty on the part of those who execute them. The object of the artist is to produce a startling_ef fect, and obtain a better sale for his pictures. The true method of taking binocular pictures for the stereoscope is described and demonstrated in Sir David Brewster's treatise, entitled, The Stereoscope : its History, Theory, and Construction, with its applicution to the Fine and Useful Arts, and to Education, chap. viii. London, 1856.

*The pictures in a room or gallery with side lights should always be viewed with the eye on which no light falls, as light diminishes the sensibility of the eye to the red rays, and therefore gives a false colouring to the picture, making all white colours of a bluish green tint.

the other, as to give imperfect vision, and by phantasy will there prevail and blot out the only remedy for this is to shut or dis- the other." This theory was to a considercontinue the use of the imperfect eye. If the able degree anticipated by M. Rohault,* image in the bad eye is very imperfect, and with this difference, that he does not suppose its degree of luminosity very small, the pa- the nerves either to cross at the commissure tient ceases to notice it, and sees tolerably or split into two. He merely supposes that well with both open. In the second case, the two optic nerves have their correspondwhen the eyes have different focal lengths, ing or sympathetic fibres, which unite in one and are equally good, a large image is united point in the brain, and join their impressions with a small one, and the effect of this is into one, thus giving a single image from sometimes to give double pictures of objects, owing probably to an effort to put aside one of the pictures. The only remedy for this defect, as we shall afterwards see, is to equalize the focal lengths of the two eyes by proper glasses.