with which they have no necessary, but merely an arbitrary connexion, established by custom and experience. So much upon the idealism of the eye.
In conclusion, we wish to hazard one remark on the subject of inverted images depicted on the retina. External objects, we are told, are represented on the retina in an inverted position, or with their upper parts pointing downwards. Now, in one sense this may be true, but in another sense it appears to us to be unanswerably false. Every visible object must be conceived as made up of a great number of minima visibilia, or smallest visible points. From each of these a cone of rays proceeds, with its base falling on the pupil of the eye. Here the rays are refracted by the humours so as to form other cones, the apices of which are projected on the retina. The cones of rays proceeding from the upper minima visibilia of the, object are refracted into foci on the lower part of the retina, while those coming from the lower minima of the object are refracted into foci on the upper part of the retina. So far the matter is perfectly demonstrable; so far we have an image on the retina, the lower parts of which correspond with the upper parts of the object. But what kind of image is it—what is the nature of the inversion which here takes place? We answer that it is an image in which not one single minimum is in itself reversed, but in which all the minima are transposed merely in relation to one another. The inversion regards merely the relative position of the minima, and not the minima themselves. Thus, the upward part of each minimum in the object must also point upwards in the image on the retina. For what principle is there in optics or in geometry, in physiology or in the humours of the eye, to reverse it? We do not see how opticians can dispute this fact, except by saying that these minima have no extension, and consequently have neither an up nor a down; but that is a position which we think they will hardly venture to maintain. We can make our meaning perfectly plain by the following illustrative diagram—In the lines of figures,
Let the line A be a string of six beads, each of which is a minimum visibile, or smallest point from which a cone of rays can come. Now, the ordinary optical doctrine, as we understand it, is, that this string of beads A falls upon the retina in an image in the form of the row of figures B; that is to say, in an image in which the bead 1 is thrown with its head downwards on the retina, and all the other beads in the same way with their heads downwards. Now, on the contrary, it appears to us demonstrable, that the beads A must fall upon the retina in an image in the form of the row of figures C; that is to say, in an image in which each particular bead or minimum lies with its head upwards upon the retina. In the annexed scheme our meaning, and the difference between the two views, are made perfectly plain; and it is evident, that if the object were reduced to only one minimum—the bead 2, for instance—there would be no inversion, but a perfectly erect image of it thrown upon the retina.
Now, there are just five different ways in which the fact we have now stated may be viewed. It is either a fact notoriously announced in all or in most optical works; and if it is so, we are surprised (though our reading has not been very extensive in that way) that we should never have come across it. Or else it is a fact so familiar to all optical writers, and so obvious and commonplace in itself, that they never have thought it necessary or worth their while to announce it. But if this be the case, we cannot agree with them; we think that it is a fact as recondite and as worthy of being stated as many others that are emphatically insisted on in the science. Or else, though neither notorious nor familiar, it may have been stated by some one or by some few optical writers. If so, we should thank any one who would be kind enough to refer us to the works in which it is to be found. Or else, fourthly, it is a false fact, and admits of being demonstrably disproved. If so, we should like to see it done. Or else, lastly, it is true, and a new, and a demonstrable fact; and if so, we now call upon all optical writers, from this time henceforward, to adopt it. We do not pretend to decide which of these views is the true one. We look to Dr Brewster for a reply; for neither his, nor any other man's rationale of the inverted images, appears to us to be at all complete or satisfactorily made out without its admission.
