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,
