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necessity to have so complicated a plexus as a retina, for this image can be obtained quite as well without it. Where would be the propriety of a double apparatus for the transmission of images when one is found to be sufficient? We have duplicates of all the senses, for a reason very distinct from that we are questioning; but, if the retina were paralysed in the living eye, neither the crystalline lens nor the foramen centrale would be sufficient to convey an impression of external imagery to the sensorium. It is the light—the direct rays--which stimulates the whole eyeball, and enables the mind to comprehend what is passing externally. This information, given in so mysterious yet simple a manner, is solely for the inward man, and not for us who are no part of him. The mind has nothing to do with the image that may be formed on the foramen centrale, or the retina as some have it ; that impressed on the cerebral ganglion is all that is requisite for its instruction.
The seeing erect and inverted images on and through lenses, is an incidental circumstance altogether; it belongs to the constitution of spherical surfaces, and is quite distinct from the operation going on within the cerebral mass at the termination of the optic nerve. An inverted image on the posterior surface of a glass globe of water, or on the animal eye, can be seen by a thousand persons at the same instant of time, yet, from their position, each eye must be stimulated by a different cone of light. Cones of light issue in every direction; and of what service could this be to the mind that wants but one impression ?
There is one remarkable fact never allowed to have any weight in our philosophy, which is, that the base of the optic nerve is not the true axis of the eye, considering it as an axis in the common sense of the word. The foramen centrale is the true axis, placing it in connection with the crystalline lens and the cornea. The optic nerve lies out of this axis, and is not at all influenced by its proximity to the foramen centrale. The part which the optic nerve has to perform is quite independent of actual imagery, such as is, or may be, represented by the accidental axis, and yet it can, and does, have an axis of its own in connection with the cornea.
This accidental axis, or focus, called the foramen centrale, is not, in reality, the axis on which vision depends ; it is simply for the purpose of collecting the rays of light which necessarily enter the eye when the external objects are represented. By bringing them to a focus, they can the more readily be absorbed, or leave the eye as spent light. The rays that strike the centre of the foramen centrale can pass back again through the axis or centre of the crystalline lens, and thus get into space again ; whilst those that diverge from the centre of this foramen centrale impinge on the choroids and are there absorbed.
The whole apparatus of the eye is simply, as has often been observed, a telescope to the seeing faculty, or, rather, to the cerebral organs of vision. If we add an artificial telescope, it is for the purpose of diminishing or increasing the size of an external object. The common day and astronomical telescopes, as well as the microscopes, effect this object, and thus assist the telescope of the eye in giving enlarged or magnified views of external objects.
When we place a lens, or globe of water, before our eye, we see an inverted image ; because the rays that issue from the external body in front of the lens do not enter the axis of the anterior surface of this lens or globe, but, coming in obliquely, undergo several reflections and refractions before they radiate from or leave the surface. Move which way we will, the same object is seen all around the posterior surface, but in an inverted position, whereas the rays that are to represent the object erect, can only be seen through the axis of the globe. It is these rays, coming straight through the axes of the lenses, that convey an erect image in telescopes ; but we must not infer from this, that vision is effected in this manner, for the rays that are to give us an erect image, do it at once by the first touch. It is the motion they induce that gives the impression.
Lenses are merely supplements to our own eye, and, whatever surface they may possess, nothing more is intended than that they shall always give a longer or shorter, a smaller or larger view of an object than can be obtained by the naked eye, as it is termed. A dead eye is, therefore, a mere lens, and it obeys the laws of all lenses having convex surfaces. This is truly and solely the whole history of the inverted image, and which inverted image, we repeat again and again, has nothing to do with the true mode of seeing.
It was not by receiving the rays from another lens in front of it, that the eye of the ox presented us with an inverted image on the little hole cut through the coats of the eye; there was no lens, nothing but a candle in front of it. But because we see the candle upside down, it does not follow that the ox, when living, must have seen it upside down, as Dr. Arnott and other physiologists would have us believe. Sir David Brewster does not believe that an object is really upside down when external to the eye; but he demonstrates that it must necessarily be upside down when the rays enter the eye. This reasoning is quite as absurd as the other ; if it were true that the rays « crossed each other at the centre of visible direction," then we ought to see the candle in an erect position on the surface of the dead eye, for, as soon as the rays crossed and recrossed, so as to reach the hinder part of the eye, it had time to turn to rights again. The ox would see it, surely, by the time it
reached the axis of the eye behind, and yet we see it inverted. Sir David Brewster proves too much when he asserts that “rays from the bottom of the image go to the upper part of the object, and those from the upper part of the image go to the bottom of the object, which process necessarily produces an erect image.”
Now this process in the dead eye, from which this strange and unsound doctrine has proceeded, has not been observed ; for if it were really true, and the retina reflected and concentrated the image, then an erect image would necessarily be seen for the very reason assigned.
The fact is, it was never intended we should see an inverted image unless a lens was placed before our eye, and only then, when it is formed by rays that fall on either side of the axis in front of the lens. When no lens intervenes, and the rays fall on our own eyes from the whole field of view, objects never can appear inverted. The mind cannot by any possibility see either the inverted or erect image that may be formed within the eyeball; it can know nothing but what the optic nerve is capable of communicating at the fibrous termination of its cerebral extremity. The impression is made from the first contact of light on the whole eyeball, and as the optic nerve is the only channel of communication, it is that duct which gives intimation of what passes without.
In what way should we ascertain that there was an inverted image on the back part of an animal's eye, but by cutting away the three coats and all the capsules? We are not sure, therefore, that there was an image there before the coats were removed. We are not even certain that the foramen centrale is the focus of all the foci which philosophers imagine exist in the retina, for we cannot impel the interior of the eye with reference to a correct knowledge of this point. But if there were a million of foci in the dead eye and in lenses, as there undoubtedly are, the mind or seeing faculty has nothing to do with them. Those foci are incidental and belong to spherical surfaces in general, and with which the mind has no connection when external objects are to be examined.
If an image is inverted on a lens, or on a plane surface, the mind sees it inverted. If rays of light were transmitted by the optic nerve after the manner agreed upon by physiologists, we should require the aid of another sense to correct the defect; for however clear the explanation given by Sir David Brewster may be to his own mind, yet very few thinking people find it conclusive. If rays cross each other as he imagines, then we still have to look at the bottom of the image, that is placed somewhere in the eye, for the top of the real external object, and the mind has to infer or gather by instinct that what is the bottom must mean the top. There is even more absurdity in this doctrine than in that part of it agreed upon by others, which imagines that we learn to know an object in its true place by the sense of touch! We cannot agree with his triumphant conclusion, that rays crossing each other, as he describes, must necessarily produce an erect image. Rays crossing each other in that manner always produce an inverted image; and if they did so present themselves to the optic nerve, the impression would always be inverted.
If we look steadily, and for a few minutes, on an inverted image that has been strongly illuminated, such as a pin resting on the ground-glass shade of a bright lamp, we shall see the pin inverted as it really appeared on the glass, the moment we shut our eyes. Whilst we are looking at the spectrum, if we push the ball of the eye we shall find that the inverted pin is stationary, proving that it is now placed beyond the jurisdiction of the eyeball. Now if the ox while living saw only the inverted image that we see on the hinder part of the dead eye, then it must always appear so to him, for no crossing of rays within his eye could rectify it, as the image is immovable on the cerebral organs of vision the very moment it reaches the foramen centrale. These are some of the phenomena of light and of lenses; we now proceed to another part of the subject.
We perceive that light is extinguished by some unknown principle in black colouring matter, and that it is decomposed and resolved into its latent state by the power which this black principle has over the particles which cause the luminousness of light. This black colouring matter is independent of the materials on or in which it exists, for whether it be the dense vapours of clouds, the flexible fabric of silk, or the solid unyielding substance of iron, the destruction or decomposition of light on their surfaces is the same. It is blackness or darkness which acts on light, and not the material in which this black colour exists, or to which it adheres.
Light, also, can be set free in a variety of ways, but principally by the contact of such bodies whose particles are so arranged as that light, as luminousness, cannot pass through their pores. Friction, concussion, and oftentimes simple pressures, disengage both light and heat; it would appear that they entered minutely into each other's composition, were it not likewise the case with other matter in a latent state. Moisture, odour, colour, &c., can be disengaged and rendered perceptible by friction, pressure, and concussion.
The sudden approximations and union of certain gaseous compounds produce light. Flint and steel, when struck together, set light free, and friction disengages it from many substances. Muscular contraction makes it visible in the lampyrus or fire-fly. There is a spontaneous emission of it, perhaps
muscular likewise, in glow worms. It is either emitted, spontaneously in decayed wood, or else the decomposition that is going on is favourable to the disengagement of light in the gaseous medium which this decomposition generates. Marine animalcules are guided in their movements by their own luminousness. Spontaneous combustion is of frequent occurrence among oleaginous and bituminous particles of vegetable fibre. A sudden blow across the eyes, disengages a greater quantity of light than the choroids can absorb-supposing it to be held in its latent state within the eye. A slight pressure is sufficient to elicit light in the eyeball, for however unacceptable the suggestion may be, yet it is in reality light which is set free by the pressure.
Light cannot be set free or decomposed by a white pigment, excepting, as before observed, when in conjunction with heat, and then it takes the character of flame. But the disappearance of fame in the fire, when the white silvery rays of the sun are shining on it, is not referable to this cause. The light of the flame cannot spread because the rays from the sun occupy that portion of space where the rays from the flame should be. Two bodies of equal character and nature cannot occupy the same space; the lesser must merge in the greater.
The particles of latent light are incorporated with all substances, consequently, it is in great abundance, much greater than that portion of it which is set free as luminousness. Like heat and all other latent matter, one particular substance does not hold it in greater quantities than another. It merely passes through certain bodies with greater ease. The particles of some bodies are so arranged as that all matter in its latent state has to travel circuitously through their interstices or pores. Other substances, differently constructed, enable all latent and perceptible matter to pass through their pores with ease, moving in straight lines.
Latent matter cannot remain in a quiescent state in any substance; for, let the density or quality of the material be what it may, it can never be set free on the supposition that it exists within certain bodies. Those substances that readily admit of the free and easy passage of latent matter through their pores, are generally the most capable of rendering it perceptible. Thus, flint and steel, two very compact, dense bodies, when suddenly struck together, compel the latent matter of light, which is traversing them, to unite with other latent matter, and thus become perceptible to us.
As was before observed, light in itself possesses no heat, not even warmth, until it fall on a substance capable of combining with it. If the sparks from the collision of flint and steel should fall VOL. XXII.-NO. 43.