. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. tory results only when, together with the successive determinationof the farthest points of distinct vision in the two principal meri-dians, the condition of accommodation of the eye underwent nochange. By his method this difficulty is removed. He proposes todefine the degree of astigmatism, by means of an astigmatic lens,the action of which can be regulated in a manner as simple as it isingenious, so as to make it assume precisely the degree by which theastigmatism of the eye is
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. tory results only when, together with the successive determinationof the farthest points of distinct vision in the two principal meri-dians, the condition of accommodation of the eye underwent nochange. By his method this difficulty is removed. He proposes todefine the degree of astigmatism, by means of an astigmatic lens,the action of which can be regulated in a manner as simple as it isingenious, so as to make it assume precisely the degree by which theastigmatism of the eye is corrected. I have had such lenses prepared,and give the description of the instrument, with the arrangementwhich appeared to me most advantageous: the principle is preciselythat of the astigmatic lens of Stokes, which name may also begiven to the instrument. It consists (Fig. 162b, exhibiting asection) of two cylindrical lenses, the one plano-convex I of to, theother plano-concave V of — ^, The first is fastened into a broadcopper ring, K, the last into K, which rings at x are fitted to one 486 K: fl another, and can turn past one another around their axis. At thesame time, therefore, the lenses IV also rotate past one another;they are turned with their flat surfaces towards each other, leavinga very small interspace. Pig. 16% A, represents the instrument,seen on the outer surface. It will be observed that on K an index ioccurs, on K a graduated scale. If the index points to 0° or to 180°,the axes of the two cylindrical lenses are parallel: the section ofthe lenses appears then as in B, so that when united, they may beregarded as a concavo-convex cylindrical lens, with equal radius ofcurvature of the two planes, whose action is about = 0. If theindex points to 90° or to 270°, the axes of the cylindrical glassesstand perpendicular to one another. At the same time the systemhas its maximum m of astigmatic action : a plane of parallel raysof light, coinciding w
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Keywords: ., bookcentury1800, bookdecade1860, bookidonanomalieso, bookyear1864
