. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, al 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
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, al 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 degrees nasal would. Fic;. 47. mean axis 75 on the ordinary trial-frame, and 15 degreestemporal would mean 105 degrees. The trial-case also contains other accessories, such asblanks or blinders, a stenopeic slit, pin-hole disc, etc., all ofwhich are referred to in the text. Combination of Lenses.—The sign of combinationis ^. Combining Spheres.—Any number of spheric lensesplaced with their optic centers over each other, and sur- OPTICS. 47 faces together, will equal one lens the value of their sum :for example, +2 S. 3 -f-1 S. ^ +3 S. will equal -f 6 S.;or a —2 S. :3 ^—i S. :^—3 S. will equal a —6 S. If a plus and minus sphere, each of the same strength, beplaced with their optic centers together, the refraction willbe nothing, for the one will neutralize the effect of the other ;for instance, +4 S. and —4 S. will be equivalent to a pieceof plane glass, as the —4 S.
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