. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. boveTable), the head was simply held to the side. It is evident that in this wayno reliable results were to he obtained. In the first place, it is certainlyextremely difficult, if not impossible, with any accuracy, to give the headprecisely the desired degree of inclination, and, in the second place, thevisual line was always directed simply towards the ophthalmometer, andwas therefore, properly speaking, not measured in the plane of a meridian,for the meridians do not cut the vis
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. boveTable), the head was simply held to the side. It is evident that in this wayno reliable results were to he obtained. In the first place, it is certainlyextremely difficult, if not impossible, with any accuracy, to give the headprecisely the desired degree of inclination, and, in the second place, thevisual line was always directed simply towards the ophthalmometer, andwas therefore, properly speaking, not measured in the plane of a meridian,for the meridians do not cut the visual line, but the axis of the cornea. A proper system of measurements seemed therefore to he obtainable only,if it were possible to make the lights themselves turn, in a vertical plane, rounda point on which the common axis of the cornea and ophthalmometer is directed,in order by so doing to make the lights shine consecutively in the differentmeridians of the cornea, while a corresponding inclination is given to the glass-plates of the ophthalmometer. To attain this object, a vertically-placed ring, Fig. •R, to whose centre the axis a a of the ophthalmometer stands perpendicular. DETERMINATION OF ITS SEAT. 4G3 and around which ring the lamps 1, 2, 3, here represented as lying in a hori-zontal line, can he turned, was steadily fixed on the ohlong table (Fig. 156,T T), between the ophthalmometer M and the investigated eye, O. Thisring, more accurately represented as Fig. 157, rests upon a pillar, S, which, Fiar. 157. by means of its bending, does not impede the turning of the lamps, and is,by a broad foot-piece, v, firmly placed upon the table. The centre of thering, c, is situated at one metre (= 3*28 English feet) from the eye ; thediameter of the ring, measured to the outer margin, amounts to 388 mm.(15-27 English inches). On this ring turn two copper-plates P P, which, at 464 ASTIGMATISM. the contact of two bent rims, r r, with the outsides of the ring, lie in thedire
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Keywords: ., bookcentury1800, bookdecade1860, bookidonanomalieso, bookyear1864
