. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, lindersurface convex. The Trial-case (see Fig. 46). —This case contains pairsof plus and minus spheres and pairs of plus and minuscylinders; also prisms numbered from 34 or i to 20 spheres are numbered in intervals of up to 2 S.;and from 2 S. up to 5 S. the interval is S.; and from5 S. to 8 S. the interval is S.; and from 8 S. to 22 S. OPTICS. 45 the interval is i S. The cylinders have similar intervals,but seldom go higher than 6 or 8 cyl. The tria
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, lindersurface convex. The Trial-case (see Fig. 46). —This case contains pairsof plus and minus spheres and pairs of plus and minuscylinders; also prisms numbered from 34 or i to 20 spheres are numbered in intervals of up to 2 S.;and from 2 S. up to 5 S. the interval is S.; and from5 S. to 8 S. the interval is S.; and from 8 S. to 22 S. OPTICS. 45 the interval is i S. The cylinders have similar intervals,but seldom go higher than 6 or 8 cyl. The trial-case also contains a trial-frame, which is usedto place lenses in front of the patients eyes. (See Fig. 47.)The eye-pieces of such a frame are numbered on theperiphery in degrees of half a circle, so that the axis of acylinder can be seen during refraction. The left of the. Fig. 46. horizontal line in each eye-piece is recognized as the start-ing-place, or zero (o), and the degrees are marked fromleft to right on the laivcr /la/f, counting around to thehorizontal meridian, which at the right hand is numbered180; this horizontal meridian is, therefore, spoken of ashorizontal, zero (o), or 180 degrees. The meridian midwaybetween zero and 180 is spoken of as vertical, or 90 some countries the meridians are differently num- 46 REFRACTION AND HOW TO REFRACT. berecl (see Fig. 48) ; for example, the vertical meridian iscalled zero, and the degrees are marked on each side ofzero up to 90 degrees. Only the upper half of the eye-pieceis thus numbered, so that when a cylinder has the upper endof its axis inclined toward the nose, the record would be somany degrees of inclination to the nasal side ; or if the upperend of the cylinder was inclined toward the temple, therecord would be so many degrees to the temporal example, in the right eye 15 de
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