The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . Fig. 7. —A, diagram of the mires alone. B, reflection of the whole instrument fromthe front of the cornea, with the arc at 180°. urement, that is, the distance between the inner* edges of thetwo mires, we know its size to be just mm. in thisinstance. Take a case in actual practice. For example, whenthe instrument is focussed properly on an eye, the black linesdividing: the mires become coincident with each other in the USE OF THE OPHTHALMOMETER L5 horizontal mer
The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . Fig. 7. —A, diagram of the mires alone. B, reflection of the whole instrument fromthe front of the cornea, with the arc at 180°. urement, that is, the distance between the inner* edges of thetwo mires, we know its size to be just mm. in thisinstance. Take a case in actual practice. For example, whenthe instrument is focussed properly on an eye, the black linesdividing: the mires become coincident with each other in the USE OF THE OPHTHALMOMETER L5 horizontal meridian (<•-/. Fig. T). This shows thai one of thechief meridians of curvature of the cornea is the horizontalmeridian. Approximate the images till they jusl touch, thenturn tin- arc at right angles to the horizontal meridian. Ifthere is n<» overlapping or separation of the images 1 and 2, itshows that the vertical meridian of the cornea has the sameradius of curvature as the horizontal. If the vertical meridian. Fig. 8. — The same images as in Fig. 7, but with the air at 0<P. has ;i shorter radius of curvature, the images will overlap to acertain extent, say, two steps (see Fig. 8). This must necessarily be so. for the size of tin- object (thedistance between the inner edges of the mires as placed on thearc) ami the distance of this object from tin- cornea havingremained the same, we must obtain a smaller image on a sur-Eace with a shorter radius of curvature. The distance between1 and T and 2 and 2 remaining the same from the constantdeviation caused by the prism, the- reduction in the size of the 16 THE REFRACTION OF THE. EYE corneal image cannot take place in a change of length of theselines. It must be brought about, therefore, by an overlappingof the inner edges of the images 1 and 2 (Fig. 8). The num-ber of steps overlapped shows the amount of astigmatism, andthe direction of the long and short indicators respectively showsthe axis at which plus or minus g
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