Skiascopy and its practical application to the study of refraction . e of the refraction has been made by theophthalmoscope or other means, quite a small mirror isfound convenient. By a large mirror is meant one from35 to 50 mm. in diameter. By a small mirror is meant oneunder 20 mm. in diameter. The mirror, or, at least, theopaque back that carries it, cannot be well reduced to lessthan 20 or 25 mm., because, if smaller than this, it willadmit light to the eye from the original source, throughthe space around the mirror; and such light, though not soannoying as a reflection at the sight hole,
Skiascopy and its practical application to the study of refraction . e of the refraction has been made by theophthalmoscope or other means, quite a small mirror isfound convenient. By a large mirror is meant one from35 to 50 mm. in diameter. By a small mirror is meant oneunder 20 mm. in diameter. The mirror, or, at least, theopaque back that carries it, cannot be well reduced to lessthan 20 or 25 mm., because, if smaller than this, it willadmit light to the eye from the original source, throughthe space around the mirror; and such light, though not soannoying as a reflection at the sight hole, is a serious hin-derance in the application of the test. The mirror platethen must be large enough to shade the eye. A large mirror having a metal cap with an aperture of 100 GENERAL CONSIDERATIONS. from 10 to 15 mm. in diameter, that can be slipped beforethe face of the mirror, or turned back at pleasure, willanswer for all sorts of testing. Such a mirror1 is shown infigure 24. As already indicated in Chapter III, the sighthole should be about 2 mm. in Fig. 24. The handle of the mirror should be rather thick, sothat a very slow even rotation can be secured ; for, as thepoint of reversal is approached, the magnified movement inthe pupil becomes so rapid that only by moving the mirrormore slowly, and making excursions of very slight extent,can this apparent motion in the pupil be readily difficulty of causing the immediate source of light tomove slowly enough is diminished in proportion as the im-mediate source of light is brought closer to the mirror. The Shade.—The shade that covers the original sourceof light should extend far enough above and below theflame to prevent the escape of any considerable amount oflight into the room. Where an argand burner is used asthe source, a cylindrical shade should be 20 to 25 cm. long,with a diameter 6. or cm., slightly greater than that ofthe chimney used, so as to allow a free current of air be-tween the sh
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectretinos, bookyear1896
