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glasses is seen through the space above the rim, through the rim, in the glasses of the spectacles, and below them. Those parts of the rim on which the rays of light do not fall, are even less of an obstruction than the part where light has accumulated. If we let the light fall on the rims of both glasses, which can be done if we stand between two windows and turn our head a little to the one side, the two lights will fall on the same spot, and being thus dense from the double quantity, the carpet or other object cannot be seen through it as when the light was single.
If we look at this bright spot on the spectacles with one eye, the other being closed, and then look at it with both eyes, the image will contract in size, and will expand again the moment one eye is closed. Every pin hole in a card will exhibit the same variation of diameter, according as we look at it with one eye or with both. This curious fact we have found was first noticed by Epinus, although the author of Discoveries in Light and Vision has recently made the same discovery without knowing that Epinus had remarked it before.
Every substance or body, whether pointed, square, or round, whether thick or thin, coloured or white, has its whole circumference edged with a thin, transparent, filmy fringe, the lines of which fringe are always, in all cases, parallel with the edge of the outline or circumference of the body, whether it be round, square, irregular, or pointed ; and when this body is of a certain diameter, about the one eighth of an inch, and is held within an inch, or less, of the eye, it is scarcely an obstruction to light or external objects. This parallel edging contracts and dilates as one or both eyes are open and rest on it. If we look at it with one eye shut, we shall perceive it to be of a certain dimension, but when we open the closed eye, a sudden contraction of the fringe or air lines takes place. The same thing occurs when looking at the light through two fingers that are nearly closed together, to the merit of which discovery our author is entitled
Now this edging must either belong to rays of light externally, or to the cerebral organs of vision; for under every circumstance and condition they are perceptible, being attached to all objects that are presented to the oblique and direct rays of light, whether from the sun, lamp, or candle. The dark and light stripes of which these air lines, as we call them, are composed, are not of stationary width, but vary with every motion of the eyelids; nor are they seen quite so distinctly when the object to which they appear attached is held before our eyes horizontally.
Every narrow slit exhibits the same phenomenon. If we slit a small portion of paper with a penknife and hold it to the light, we shall see these air lines very plainly; and by looking at them
first with one, and then with both eyes open, we shall see the contraction and dilatation of the slit. In this way we can perceive that the dark and light air lines are not always in one place, and whilst looking at them another curious phenomenon is made visible. Throughout the slit or opening all the air bubbles which the aqueous humour contains are plainly seen, but instead of being round, as when viewed through a pin hole, and described in " Discoveries in Light and Vision,” they are flattened or lengthened in the direction of the air lines ! If we look at them when the slit is held upright, they are long and narrow; if we hold the slit horizontally, they are still of the
. same shape, except that they are elongated horizontally. This phenomenon we have also observed on that part of the rim of a pair of spectacles on which light is condensed.
Whilst we are examining this singular phenomenon, as we before observed, an object external to these air lines, can be seen through the rim of the spectacles nearly as well as if the rim was not there. Not only light is perceptible, but objects. Now it is obvious that light cannot pass through any dense substance, such as the dense rim of silver; how, therefore, is it, that on looking at the rim, we see the carpet or window sash, or any other object, almost as well as if there were no rim before our eye?
If we take off the cover of a wafer-box or pill-box of paper, and wind a few strans of coarse silk over the open part, making a pin hole in the bottom, we shall see a very curious phenome
To make the experiment perfectly, we should stand with our back to the window, and whilst holding a piece of white paper a few inches from the pin hole behind, look at the bars of silk, which we can plainly see even when the box is held near to the eye. On first looking at the box the bars of silk will be plainly seen, and on moving the box a little to the right or left the pin hole will be also visible, but very much magnified, and, in the very centre of one of the bars of silk the pin hole will appear as perfect as if no bar was there.
What is more extraordinary, in the centre of this pin hole another bar will be seen of smaller size than the real bar.
Now it will be admitted, that the rays from the magnified pin hole and the distant bar could not penetrate the thick thread of silk-magnified, apparently, to half an inch. The bar of silk is impervious to light, and yet there is a medallion of light, having a bar in its very centre, which medallion appears set in the very middle of the real bar of silk.
This part of the phenomenon is referable to the same laws which allow us to see objects through the air lines of a pair of spectacles. The pin hole is a lens of great magnifying power, which, having thrown its rays on the bar of silk on the surface VOL. XXII.-NO. 43
nearest to it, receives them back again. These rays, thus carrying the image of the silk, pass on each side of the real bar on which our eye rests, and converge to the lens of our own eye. The pin hole and the bar are therefore in this way transmitted to the cerebral organs of vision.
We know that many a curious theory will be struck out of this new phenomenon, but the above that we have ventured to offer has simplicity to recommend it, as we need no recourse to refractions and reflections not common to vision. In the case of the inverted image on the hinder part of the eye, seen in optical experiments and on all globular surfaces, facts no way belonging to vision are produced to support an absurd theory. To comprehend our views fully, we shall beg leave to explain a few of the phenomena attached to lenses, and then, after endeavouring to throw some new light on the subject connected with vision, bring our remarks to a close.
We cannot, by all the experiment that can be brought to bear on this subject, attribute the alternate dark and light stripes of these air lines to any interference in rays of light. It appears to us to be but a multiplication of outlines of the body to which they are attached; but of this on some future occasion.
Lenses, whether concave or convex, show both the erect and inverted image on their surface. When a convex lens is at a little distance from us, the object in front of it is seen in an inverted position on the back surface. If the lens is brought close to the eye, then the rays from the object in front come through the lens in a straight line to the eye, and the figure is erect.
In page 39 of the work under review, it is stated—and we all know the statement to be correct--that, if we look at the glass globes in an apothecary's window, when inside of the shop, we shall perceive that all the objects in the street are inverted on these glass globes. But if we look through them, so that the central rays pass through the axis of our own eye, the same objects will be erect. This takes place whether the lens be large or small, thick or thin, solid or filled with fluids; whether it be the eye of an animal, or one of glass.
Convex and concave glasses, or lenses of different focal lengths, placed at suitable distances from each other, as in the coinmon day telescope, show the external objects erect, and likewise show an erect image of them when looking on the surface of the eye-glass. If we take out two of the lenses the external objects will be seen in an inverted position.
If two glass globes, filled with water, touch each other, and an illuminated object is placed on a line with them, so that the axes of the three are parallel, the illuminated object will be erect on the front of the glass the farthest from it.
The eye of an ox, when parallel with the axis of our own eye, and with that of an illuminated object in front of it, shows an erect image; or, rather, the rays of the external object pass through the dead eye, and represent the object on our own eye in an erect position, just as the rays of an illuminated object cast an erect image on the surface of the second globe of water. The dead and the living eye, as it respects the image which may be perceptible externally, are, in reality, nothing more than two globes filled with a transparent fluid. If the dead eye be removed from its parallel position, then the rays from its axis fall elsewhere, and we merely see the oblique rays on the back surface, which rays produce an inverted image.
It is, therefore, owing to the manner in which lenses are placed, with regard to the parallelism of their axes, that an external object is represented erect or inverted. The position of an object is owing to refractions and reflections that are not at all connected with the true mode of seeing.
An interposing lens is an intrusion, and prevents our seeing the object that would otherwise be seen if the lens were not thrust before our eye. We should recollect that the object is lost to our vision by the interference of the lens, for the focal power of this lens is very different from that of our eye when it is alive, and we have command over its muscles and nerves.
It cannot be too often observed that, when an eye is taken from the socket, it loses all power save that which it has in common with every lens—the power of transmitting lightwhether it be filled with a fluid, or it be a solid piece of glass. The retina of a dead eye is of no more importance to the image that is seen on the little hole behind, than if it were the sclerotic coat.
Our own eye enables us to see large objects either far off, or near to us; whereas an artificial lens, or the globe of a dead eye, gives us but a very diminutive image of the object placed in front of it. A lens of a foot diameter can only magnify the external object to the size of two or three inches at the distance of twenty or thirty feet; and our own eye, scarcely an inch diameter, shows us the full size of the object.
The eye of an ox, or of a man, is nothing more to its owner than a telescope, even when it is alive, and the will has power over it. When
When it is in our hand for experiment, it is a simple, globular lens, containing two fluids of different refractive powers. The fact of seeing the inverted image on its posterior surface is not indicative of the true mode of seeing, for that depends on the impression which the extremities of the optic nerve make on the cerebral ganglion of vision.
We have observed that the rays from an illuminated object, when transmitted through two globes of water, show an erect
image on the front surface of the second globe. The question might then be asked, why an erect image is not seen on the hinder part of the eye, perceiving that it is composed of two lenses-two convex lenses? The cornea, and the little opening behind, form a convex lens, and, between these anterior and posterior surfaces of the eye, there is another lens called the crystalline. But, in answer to this objection, it must be recollected that, after all, they really form but one lens, for the anterior surface of the crystalline is convex to the concave surface of the cornea, and the posterior surface of the crystalline lens is convex to the concave surface of the little hole that is cut behind the eye. On the contrary, if the crystalline lens were taken out and placed outside of either surface of the eye, there would then be two convex lenses.
The eye of an ox is, therefore, a simple convex lens filled with fluids, and our living eye-confining our remarks to images . seen on the surfaces--is likewise a convex lens. When our eye is not parallel with the eye of the ox, then we lose sight of the direet rays that would have passed through our axis, and only see the rays that fall obliquely on the animal's eye. When the eye is raised on a line with our own, then we see the central rays. In this case, our eye and the animal's eye are strictly analagous to the two globes of water placed parallel with an illuminated object.
If we interpose a small glass globe, filled with water, between our eye and an illuminated object, a bystander will perceive that the object is erect on our own eye, and yet it is inverted on the glass globe if we look on the posterior surface. In general, a candle is resorted to for an experiment of this kind, but, owing to the intensity of the light, there are but few who can bear the pain. Standing before the window with a pin in front of the glass bulb, is quite sufficient to test the fact. The pin, though but a small object, is seen distinctly on the cornea by any one having a good eyesight.
If we can dispossess ourselves of the erroneous notion that a retina is requisite to the appearance of those images seen on the little convex hole behind the dead eye, we shall soon come at the truth. We shall then perceive that the eye of an animal in our hand shows us nothing more than what can be seen on, and through, a little glass bulb. Very few can make the experiment with the eye of an animal, for unless, as our author observes, the eye is perfect and fresh, and have been delicately handled, the aqueous and vitreous humours run into one another, and it frequently happens that, by the rough handling of the butcher, when taking the eye out, the crystalline lens has slipped out of its capsule-a fact which has been ascertained by dissection.
For the mere purpose of producing an image, there is no