In Nothoceras, the bent edges of the Septa (the goulot of the French palæontologists) protecting the Siphuncle, instead of being deflected backwards as in Nautilus, Cyrtoceras, &c., are deflected forwards, or towards the opening of the shell, as in the Ammonites. ON SIR DAVID BREWSTER'S SUPPOSED LAW OF VISIBLE DIRECTION. BY THE REV. GEORGE PAXTON YOUNG, M. A., Read before the Canadian Institute, March 7th, 1857. Sir David Brewster claims to have proved experimentally, that, in monocular vision, whatever be the direction in which a ray strikes the retina, it gives the sensation of vision in a direction perpendicular to the retina at the point of excitement. This is his Law of Visible Direction. A careful examination of the eye has shewn that the retina and the cornea have a common centre (which may, therefore, be conveniently termed the centre of the eye), and that a normal to the retina at the point where the picture of a small visible object is formed, almost exactly coincides (at least in pencils of moderate inclination to the axis of the eye) with the line joining the centre of the eye and the object; so that according to the Lawof Visible Direction, a small object is seen in the direction (nearly) of a line drawn from it to the centre of the eye. From this law of visible direction in monocular vision, has been derived a corresponding Law of Visible Position in binocular vision; which is, that a small object seen with both eyes, appears at the point where the lines of visible direction for the two eyes meet; the meeting of these lines being a condition indispensable in order that the object may be seen single. These laws, while admitted by some philosophers of high authority, have been called in question by others, though I have never seen any thing like a satisfactory refutation of the arguments advanced by Sir David Brewster in support of his theory. I agree with those who deny that Sir David's reasoning is valid; and I propose in the present communication to shew that the experiments on which he relies are quite inconclusive; in doing which, it will be sufficient to discuss the case of monocular vision; for, since the law of visible position in binocular vision is professedly derived from that of visible direction in monocular vision, it follows that if the latter be destitute of evidence, the former must be given up likewise. Sir David Brewster has no where formally explained what he means by visible direction; at least he has not done this in those papers in the Philosophical Magazine, which are expressly devoted to the proof and illustration of his Law; in consequence of which, the real import of the Law is involved in considerable doubt. But probably Sir David would accept the following as a true statement of what he holds, viz that the mind, being mysteriously united with the retina as part of the living organism of the body, is immediately cognizant of the affections excited in the retina; and that it refers the affections of which it is thus cognizant to a stimulus situated in the direction of a normal to the retinal surface. A writer in the Athenæum for February 7th, of the present year, thus states what he supposes to be Sir David's theory: "The mind, residing as it were in every point of "the retina, refers the impression made upon it to a direction coin"eiding with the last portion of the ray that conveys the impression.' This is undoubtedly a mistake. Instead of: refers the impression to a direction coinciding with the last portion of the ray that conveys the the impression, the statement should lave at least been refers the impression to a direction perpendicular to the retina at the point where the refracted ray falls upon its surface. With this alteration, the sentence quoted would substantially agree with what I have expressed. Now it is important to observe at the outset, that, even if it be true that the mind "residing as it were in every point of the retina," or, to use a less objectionable mode of expression, mysteriously united with the retina as part of the living organism of the body, is immediately cognizant of the retinal affections, this is a metaphysical truth, which does not admit of being experimentally demonstrated. It must be established by its proper evidence: and this is of itself enough to shew that Sir David Brewster, in fancying that he has experimentally proved his law of visible direction, must be labouring under some delusion. From the nature of the case, physical experiments are inadequate to establish a law whose necessary basis is a metaphysical principle. Passing this, however, let us proceed to examine Sir David Brewster's experiments The following is perhaps the most beautiful and plausible of the direct experiments on which he relies in support of his Law: "Having expanded the pupil by belladonna, look directly at a point in the axis of the eye. Its image will be formed by a " cone of rays variously inclined from 85° to 90° to the surface of "the retina. While the point is distinctly seen, intercept all these "different rays in succession, and it will be found that each ray gives "vision in the same direction, the visible point retaining its position. "Hence it follows, that on the part of the retina in the axis of vision, "all rays, however obliquely incident, give the same visible direction "perpendicular to the surface of the membrane." Now, I admit that a very interesting fact in vision is here proved: and let Sir David Brewster have the credit of having established it. But what is the fact proved? It is, that all rays falling upon the part of the retina which lies in the axis of vision, give rise to the same subjective affections, whatever be the inclination at which they impinge upon the retina. Nothing else than this is made out. Sir David Brewster indeed thinks, that, because the visible point retains its position while the different rays in succession are intercepted, we are warranted in affirming that "each ray gives visible position in the same direction." But what is meant by the visible point retaining its position? There does not exist any visible point, or image, to which position in absolute space, apart from the mind, can be ascribed. When a visible point, therefore, is said (popularly) to retain its position, the idea really intended to be conveyed, must be, that no appreciable alteration is experienced in the subjective affections of which we are conscious. If we refer (as we are under no necessity of doing) our subjective affections to a remote stimulus, it is of course to be expected, that, while no appreciable change takes place in the subjective affections, no wavering or variation shall occur in the estimate which the mind forms of the direction of the stimulus. But the circumstance of "the visible point retaining its position" indicates nothing whatever regarding such reference, whether determinate or variable. A visible point is a subjective phenomenon. A change in its position is a change occurring in a subjective sphere. The absence of any change in its position is the absence of (appreciable) change, in a certain respect, in our subjective affections. The experiment described merely shews, therefore, that all rays falling upon the part of the retina in the axis of vision give rise to the same subjective affections; and hence it has no weight in demonstrating the law in support of which it is adduced. But besides failing to observe that the circumstance of the visible point retaining its position while the different rays in succession are intercepted, amounts to no more than this, that rays incident upon the same part of the retina at different obliquities give rise to 66 the same subjective affections, Sir David Brewster has committed the glaring impropriety of assuming that an object in the axis of vision is seen in the direction of the axis. For how does he argue ? “Each ray gives vision in the same direction, the visible point retain"ing its position." Let this sentence pass, dubious as it is; and what next? "It follows, that, on the part of the retina in the axis "of vision, all rays, however obliquely incident, give the same "visible direction, perpendicular to the surface of the membrane." Indeed! How does this follow? Grant that the rays in question all give the same visible direction (though the only thing proved, is, that they give rise to the same subjective affection); how does the idea of a direction perpendicular to the surface of the membrane creep in? The cone of light through which vision is produced, contains a line of rays, no doubt, which fall perpendicularly upon the eye, and pass to the retina without refraction; and it may be fancied that these at least give visible direction" in the axis of vision. But how can such a thing be proved? How does it appear, that, when rays come to the eye along the axis of vision, the mind determinately refers the subjective affections cecasioned by such rays to a remote stimulus, situated somewhere in the axis? Let E represent the eye, and O an object towards which the axis of the eye is turned. It may perhaps be said, that, if you ask the observer, he will tell you that he refers, and cannot help referring, his sensation to a stimulus in the line E O, But he means nothing more by this, than that he is unable, while his eye is turned towards O, to alter the character of the perception realized. That nothing more than this can be intended, and that there is not, in truth, any intuitive or instinctive reference to the direction E O, is rendered certain by a consideration which shall afterwards be more fully brought out, viz: that the object O is not an object of intuitive knowledge at all. Distant objects can only be known mediately or inferentially. And if the object O be not immediately known even as existing, it follows, a fortiori, that the direction E O is not immediately known; so that an instinctive, intuitive or immediate reference of a visual impression to the direction E O, is an absurdity. This may suffice, as regards direct demonstration. Ex uno disce omnes. No direct demonstration can possibly indicate any thing else than the similarity or dis-similarity (as the case may be) of the subjective affections produced by rays impinging upon particular parts of the retina. Let us proceed to consider next whether Sir David receives any more effectual support for his doctrine from the indirect method of proof-the method which, beginning with a certain hypothesis, and deducing the results to which it leads, concludes from the harmony between these results and actual fact, that the hypothesis is correct. And here again, as I intend to limit myself to a single example, I shall choose the most elegant and specious that I can find. Many writers on vision have perplexed themselves with the enquiry: why are objects seen erect, when their pictures on the retina are inverted? Sir David Brewster tells us that this is a necessary consequence, and therefore a confirmation, of his Law of Visible Direction. "The phenomenon," he writes, "of an erect object from an "inverted picture on the retina, which has so unnecessarily perplexed "metaphysicians and physiologists, is a demonstrable corollary from "the law of visible direction for points. The only difficulty," he adds, "which I have ever experienced in studying this subject, is, "to discover where any difficulty lay." In examining this statement, I would repeat the remark previously made, that the image or "phenomenon" of an object has no existence in absolute space, apart from the mind. No doubt, the language familiarly employed in treatises on vision tends to suggest a contrary idea to careless and unreflective readers; and few philosophers are at less pains to avoid phraseology liable to be misunderstood, than Sir David Brewster himself. He not only at one time, tells us of an image being formed in front of a wall, or behind a wall, according to the circumstances of the experiment; and, at another time, speaks of images floating in the air at a distance of so many feet from the eye; but he even accuses certain images of assuming a position in space different from "their right position " But, of course, such languagewhatever be its meaning-cannot signify that images do ever actually exist in space, apart from the mind. I do not affirm that images are purely subjective states: modes of the ego considered per se, and out of all relation to matter: modes in which the ego might have existed, though matter had never been. Most metaphysicians take this view. A different opinion, however, may be maintained. it may be held that an image is not a purely subjective state, but is constituted by the mind's immediate apprehension of the non-ego; that it is a product of two factors, the mental and the material, mysteriously nited with, or existing in relation to, one another. Being desirous to avoid metaphysical discussion as far as possible, I shall not attempt here to judge betwixt these two opposite theories. But, whether the one or the other be correct; whether an image be purely subjective, or partake partly of the subjective and partly of the objective; this |