Page:Handbook of Ophthalmology (3rd edition).djvu/113

Rh progression as the correcting lens is withdrawn from the eye. The most certain method, that of Coccius, is to hold the correcting lens close to the eye under examination; but just in that case the difference between the images in apparent myopia and in actual myopia, caused by elongation of the eye, is least apparent.

Still more important is the fact that we possess no means of measuring the size of the upright image, but can only estimate it approximately. The errors likely to be made under such circumstances may be shown by a simple experiment. Any object, say the smallest size of ordinary print, is placed at the focus of a convex lens, provided with a diaphragm, which must not be too small. If now the observer place his eye close to the lens, he perceives a visual field of a certain extent, and the objects in it magnified to a certain size. If now he withdraw his eye from the lens, he can scarcely avoid the impression that the objects become more magnified. Evidently this is an optical illusion, based upon the fact that the relations between the enlargement of the objects and the size of the visual field change to the disadvantage of the latter. The object portrayed upon the retina of the observer is, however, the virtual image of the test letters at the focus of the convex lens; but this image lies already at an infinite distance behind the convex lens, and accordingly, as regards the size of the retinal image, the withdrawal of the eye a few inches from the lens has no effect. If we prefer to assume, with Mauthner, a common optical centre for the eye and the convex lens, it is of course true that this point recedes from the retina the more the distance between the eye and the convex lens is increased; but at the same time the distance between this point and the test type behind the lens increases in the same proportion, so that the size of the retinal image is unaffected. That this last must be the case we have already proved by Fig. 25.

We have hitherto spoken only of the size of the retinal image. Generally the process has been another one, and the attempt has been to determine not the visual angle, but the enlargement, assuming a so-called "distance of distinct vision," of 8 inches. From the present stand-point of ophthalmology it seems very desirable to dispense altogether with such an antiquated idea as that