. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, faster in onemeridian than in the meridian at a right angle. The astig-matism is in the meridian of slow movement. The apparent difference between the plane and con-cave mirror in the direction of moxement of the retinalillumination.—With the plane mirror the rays of light as if they came from a point just as far back of 164 REFRACTION AND HOW TO REFRACT. the mirror as the original source of Hght is in front of surgeons eye behind a plane mirror is,
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, faster in onemeridian than in the meridian at a right angle. The astig-matism is in the meridian of slow movement. The apparent difference between the plane and con-cave mirror in the direction of moxement of the retinalillumination.—With the plane mirror the rays of light as if they came from a point just as far back of 164 REFRACTION AND HOW TO REFRACT. the mirror as the original source of Hght is in front of surgeons eye behind a plane mirror is, therefore, inthe path of these rays, and sees that portion of the pupil-lary area illuminated to which these rays are directed.(See Fig. 146.) With the concave mirror the reflected rays come to afocus, forming an inverted image of the flame, which be-comes the immediate source of light /;/ front of theobservers eye. When the concave mirror is tilted, theimmediate source of light goes in the same direction, butwith the result that the opposite portion of the pupillaryarea is illuminated. (See Fig. 143.) This shows the. Fig. 146. immediate source of light at L and mirror tilted downward ;the rays proceeding from L diverge and illuminate theupper portion of the pupillary area. Tilting the mirrorupward, the immediate source of light at L moves upwardalso (L^), and the lower portion of the pupillary area be-comes illuminated. (See also Fig. 142.) Rule for Neutralizing Lenses with the Plane Mirror.—When the retinal illumination appears to mo\e in thesame direction as that of the mirror, the observer is withinthe point of reversal and a plus lens must be placed beforethe eye to .stop all apparent movement. When the retinalillumination appears to move in the opposite direction to KETINOSCOPV. 165 that in which the mirror is tilted, the obsener is be)ondthe point of reversal, and a minus lens must be placedbefore the e\e to stop all apparent movement. Rule for Neutralizing Lenses w
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