. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, 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 the
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, 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. will diverge rays of light asmuch as the +4 S. will converge them, and the result. Fig. 48. is, rays of light parallel before refraction are parallel afterpassing through such a combination. If, however, a plusand a minus sphere of different strengths are placed together,the value of the resulting lens will equal their difference, infavor of the higher number; for instance, +4 S. and —2S. will equal a +2 5., the —2 S. neutralizing 2 S. of the+ 4 S., leaving +2 S. Combining Cylindric Lenses.—Any number of c)lin-dric lenses placed together, with their axes in the Si7;j/c 48 REFRACTION AND HOW TO REFRACT. meridian, are equal to a cylinder the value of their sum ;for example : +2 cyl. axis 90 degrees and -f 3 cyl. axis 90degrees will equal a -f 5 cyl. axis 90 degrees ; or —2 180 degrees and —3 cyl. axis 180 degrees will equala —5 cyl. axis 180 degrees ; or —2 cyl. axis 180 degreesand +1 cyl. axis 180 degrees will equal a—i cyl. axis180 degrees. As a cylinder refract
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