Page:Ferrier's Works Volume 3 "Philosophical Remains" (1883 ed.).djvu/354

344 one-third of an inch high, and very much smaller than the retina, represented by the spectacles—is this tower another tower, seen to be a hundred feet high, and infinitely larger than the retina, and existing out of the mind in rerum natura? or is not the latter tower merely suggested by the former ideal one, in consequence of the great disparity which touch, and touch alone, has proved to exist between the thing seeing and the thing seen? Unquestionably the latter view of the matter is the true one; seen objects are always ideal, and always remain ideal; they have no existence in rerum natura. They merely suggest other objects of a real, or at least of a tangible kind, with which they have no necessary, but merely an arbitrary connection, 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