. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. t vision upon a darker ground (forexample, on black velvet). These phenomena are connected with the com-position of the lens of so-called sectors, which, as Helmholtz showed, canbe very well seen with the magnifying-glass, with lateral focal illumination,in the living eye. All these irregularities of the lens increase with thetime of life, and partly explain the diminished acuteness of vision. Of all the entoptic objects observed, we can easily determine the positionin point of de
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. t vision upon a darker ground (forexample, on black velvet). These phenomena are connected with the com-position of the lens of so-called sectors, which, as Helmholtz showed, canbe very well seen with the magnifying-glass, with lateral focal illumination,in the living eye. All these irregularities of the lens increase with thetime of life, and partly explain the diminished acuteness of vision. Of all the entoptic objects observed, we can easily determine the positionin point of depth, according to a method, to which I was led by those of SirDavid Brewster and of Listing. Instead of looking through one, we lookthrough two openings of 0-1 mm., placed at from 25 to 3 mm. from eachother. Two cylinders of light (Fig. 104, a a, b b, and c c, dd)y each in itselfhomocentric, then penetrate the eye, under such an angle, that the circles ofwhich a b and c d, are the diameters, on the retina cover one another nearlyby half. We therefore see them as Fig. 105. In each circle the entoptic Fig. Fig. 105. \ ?• ooii .. / 202 MODIFICATION OF THE ACUTENESS OF VISION. spectra are now to be seen. From a point 1, situated in the plane of thepupil i i, the two entoptic shadows lie precisely in the middle of the twocircles, in the circle a b, at c, in the circle c d, at b, and therefore preciselyas far from one another as the centres of the circles themselves : for eachother point, situated in the plane of the pupil, the mutual distance of thetwo entoptic shadows, although now falling in other parts of the circles, isof course equal. On the contrary for a point 2, situated in the cornea, theyfall farther from one another, as 2 and 2; for a point 3, situated behind theplane of the pupil, they fall closer to one another, as 3 and 3, as the linesdrawn from these points, parallel to the rays of the two cylinders, now see also, without further demonstration, that
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