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In the received doctrines of light, therefore, it must be apparent that all that rests upon observed facts and laws, carried out and applied by geometry, is beyond controversy ; and such, most unquestionably, are all the ordinary productions of images by mirrors, and lenses and many kindred phenomena: on the contrary, all which belongs only to the regions of conjecture or speculation; as for instance, all questions relating to the forces which influence light in reflection or refraction, or to the modes in which light is essentially developed, &c. ; is liable to be disputed by any body, and without the possibility of ever bringing the controversy to an absolute issue. In our analysis and examination, therefore, of the new treatise on light and vision, whose title stands at the head of this article, we propose a strict adherence to the acknowledged laws of investigation; reasoning only from established facts, and observed properties, and applying to these the certain aids of geometry.
The first and most important point in which this writer opposes
the received doctrines of light, relates to the inverted image formed on the retina over the posterior surface of the chamber of the eye; and the author's proposition is thus announced:
“ Having always doubted the theory of seeing objects in an inverted position, I determined on testing its accuracy. The first moment it was in my power, I made the experiment, and the first eye that I examined enabled me to detect the error under which we had so long labored." p. 1.
Then follows a description of the preparation of an eye, in the usual way, by removing the sclerotic coat, &c. from the hinder part of the eye, and a statement of the commonly observed fact of an inverted image on the posterior surface. But supposing this inversion to result from some derangement in the eye consequent on its removal from the socket, or from some other cause different from that usually ascribed, the author substituted an artificial pressure, similar to that of the socket, by “ holding it in the hand as if it were a tube, and pressing it all round equally yet gently."
"Instead of looking on the surface of the hole as is usually done when making this experiment, I looked through it; because, in the case of our own vision, I knew that the rays of light from an external object had to pass through the axis of the eye. At first nothing was seen, even the inverted candle had disappeared from the surface of the
le. Presently a bright light flashed across the animal's eye, and disappeared. As soon as I could keep my hand steady, the light again was visible; it proved to be a circular, and bright; and while it was thus stationary, I held a pin before the eye, with the head up, and to my surprise and pleasure, I found my previous opinions confirmed, for the pin appeared exactly as I presented it to the eye of the animal. When I turned the pin upside down, it appeared upside down too! As soon as I lowered the eye, so that the central rays from the candle did not enter the axis of the animal's eye, then the candle again appeared on the surface of the little hole.”p. 4, 5.
On this first and single experiment, and the result as above announced, the writer most positively asserts, that the doctrine of the inverted image is “a gross delusion,” and that the rationale heretofore adopted as explaining the fact that objects appear to us erect while their images are inverted, “will in a quarter of a century be the subject of amazement, ridicule, and merriment.” We must remark upon these assertions, that they are, in our opinion, unsupported altogether by the facts of the case; and though we certainly feel a decided sentiment of respect for this bold, ingenious, and original writer, yet we could not but be surprised and amused at the degree and manner of the temerity, that could speak so scornfully of the results which observation and mathematical principles have been held to concur in establishing in regard to the doctrine of inverted images, and the ingenious, if not strictly true account which has been given of the way in which we see objects erect, while the images are inverted on the retina.
We consider it a fact too strongly supported by experiment, and too decidedly indicated by geometry, to be lightly yielded, that there is formed upon the back part of the chamber of the eye, an inverted image of every object within the range of vision; and, although the writer before us is so confident in asserting the contrary, we cannot, without more abundant evidence, and more satisfactory explanation, renounce the wellsustained doctrines of vision which now obtain. At the same time, we are fully willing to dispense in this case even-handed justice, and to give the author credit for some good, if not original suggestions, relative to the mode of action of some of the more delicate organs of sight. For full two centuries, have anatomists and philosophers been examining with great care the structure of the eye, and the mutual connections of its parts. These examinations have, indeed, been accompanied by various opinions as to the exact functions of each division of the organs of sight; yet, to this one conclusion, all who have closely examined, have uniformly come, viz. that the eye is justly considered a natural acromatic instrument, or camera obscura, in which pictures of external objects are exhibited as painted on the retina, or posterior surface of the chamber, by rays introduced through the apurture of the pupil. That the eye is constituted, and its parts arranged, with a special design
to the formation of images, is among the most indisputable of all propositions in this department of optics. Indeed, this single fact is enough to convince a person with sufficient knowledge of geometry to understand the case-viz., that the transparent front, or cornea, is found to possess exactly that shape which is demonstrably effective in preventing what is called spherical aberrations, and bringing the rays of light precisely to a focus; while, at the same time, the relative curvatures of the two surfaces of the crystalline lens, and the relative densities of the two humors, and of this lens, are found to be in all cases suited, with the utmost precision, to the purpose of forming images nearly at the place of the posterior surface of the eye. These facts, it must be acknowledged, are of a character not easily to be set aside in investigations on this subject.
Again : as to the position which the image, if formed, must have. It is a proposition of the most elementary simplicity-that a ray of light so falling on any double convex, or double concave lens, as after refraction to pass through the central point of the lens, must assume a direction again on passing out, parallel to that in which it fell on the lens; and when the lens is not very thick, the effect of this law will be to occasion a crossing almost exactly at the centre of the lens, of the rays from the extremity of an object. From this arises the necessity of inversion in the images produced by double convex lenses. Now, on examining the eye, and noting its peculiar structure, who can, fail to be struck with the position and form of the crystalline lens and its density, as compared with the other divisions of the organ—and who does not see the obvious propriety of the inference from analogy, that the rays of light proceeding from the extremities of objects, cross each other near the centre of the crystalline, and so produce an inverted image somewhere beyond; or, as the arrangement and refracting powers of the several parts, indicate, about the place occupied by the retina on the posterior surface. This inference, so deduced from analogy, and from reasoning a priori-has been considered as beautifully confirmed by the celebrated discovery of the German philosopher, Scheiner, in the former part of the seventeenth century. "By taking the eye of an ox recently killed, and stripping the sclerotic coat with the choroides, from its posterior portion, carefully preserving the retina as it lies upon the vitreous humor; then placing the eye in a suitable aperture in the window-shutter of a darkened chamber, with the cornea outward, a transparent miniature painting of the external landscape, in all its variety of figures and colors, is exhibited on the retina, the whole being inverted.” The inversion, as we have seen, was of course regarded as the natural optical effect of the structure of the eye. Yet the above experiment which offered, as it were, so palpable a demonstration of the fact, directed the attention of philosophers to the question, how it is that we nevertheless see all objects erect-and called forth numerous disquisitions, mechanical, optical, and metaphysical, to account for the phenomenon. In this controversy, are to be found the most illustrious names of that and of subsequent times.
The most approved and generally adopted solution of the circumstance, is that which the writer before us would have us believe, is about to become a subject of amazement, ridicule, and merriment.” It is happily illustrated by Dr. Arnott. “It is known,” says he, “that a man with a wry neck judges as correctly of the objects around him as any other person, never perceiving them for instance, inclined or crooked, because their images are inclined as regards the natural perpendicular of his retina ; and that a bed-ridden person, obliged to keep the head upon the pillow, soon acquires the faculty of the person with a wry neck: and that an affected girl, inclining her head while trying her attitudes, from much practice, judges of the manæuvres of a beau, as conveniently in that way as any other; and that boys who, at play bend down to look backwards through their legs, although a little puzzled at first, because the usual position of images on the retina is reversed, soon see as well in that way as in any other. It appears therefore that while the mind studies the form, color, &c. of external objects, in their images projected on the retina, it judges of their position by the direction in which the light comes from them to the eye, no more deeming an object to be placed low because its image may be low in the eye, than a man in a room, into which a sun-beam enters by a hole in the window-shutter deems the sun low because its image is on the floor.”
We cannot maintain the absolute certainty of this solutionbecause the case in its very nature does not admit of that kind of examination, which alone can lead to absolutely certain results. As to tracing the exact mode in which any of our organs convey impressions to the sensorium--and determining without the possibility of error the precise way in which originate our perceptions of external things, and our judgment about themwe hold it to be a point excluded by the very limitations of our constitution. All we can expect in the case, and all we need aim at, is to examine as closely as we can each special organization, and all the circumstances connected with the same, and then fairly draw our inferences as to the probable and natural mode of action. This course may be most ably, and wisely pursued, and yet, in certain cases, error may not be avoided, either through an imperfect apprehension of the proper offices of some parts under examination, or through ignorance in relation to some collateral branches of knowledge. We will not assert that such may not possibly have been the case with the received doctrines of light and vision. But this we will assert, that those doctrines, so far from even meriting the ridicule here predicted, will always stand as admirable monuments of philosophical research and sagacious inference. We take occasion too, to add here, that while in the main, the supporters of the established doctrines of vision, have been guided by the unerring indications of geometry--the writer before us manifests, if not a disregard for this guidance, certainly a most unpardonable ignorance of its powers and adaptation to the subject.
We have said that we cannot renounce the well authenticated facts and conclusions touching the established theory of vision on the bare assertions and inadequate investigations of this writer; at the same time, we do not question some of the facts here brought forward. We do not doubt, for instance, that after preparing the eye under examination as described, the author did, under certain circumstances, lose sight of the inverted image, and perceived a confused and nearly circular image not inverted. Nor do we deny that it is possible to see through an eye in some given positions, tolerably well defined objects as they are, erect; and this too, possibly, when they may be beyond the focus, proper for allowing the dead eye, regarded as a simple lens, to present the rays to the observer's eye under the best angle for distinct vision. But we believe a ready explanation may be rendered of all these cases entirely consistent with the received doctrines of light and vision.
If we look through any double convex lens, we shall find (if our eye be held close to the lens, the only case in which we can really look through it) objects which are about the focus of the lens, much magnified and erect-and more distant objects, will appear erect also—and less and less distinct as they are more and more distant from the focus of the lens. The reason of all this is abundantly manifest by the simplest application of geometry to the Newtonian theory. Yet, it is to be remarked, there is no image formed in these cases, by the lens held before the eye, but the convergent rays are received by the eye, before they have reached their focus. And the very course pursued by the writer under consideration, indicates to us that the animal's eye used in the experiment referred to, after being mutilated and pressed, as stated, would most likely be incapable of VOL. I.-NO, I.