. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, front of hiseye, so that the reflected rays entering the e)e are returnedtoward the mirror. There is an infinite variety of these in-struments in the market, but for the general student themodified instrument of Loring appears to meet with mostfavor. (See Fig. 79.) This has a concave mirror with a radius of curvature of40 cm., giving a principal focus, therefore, at 20 cm. Thesight-hole is round and about 3^ mm. in diameter, cutthrough the glass ; this mirror can be til
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, front of hiseye, so that the reflected rays entering the e)e are returnedtoward the mirror. There is an infinite variety of these in-struments in the market, but for the general student themodified instrument of Loring appears to meet with mostfavor. (See Fig. 79.) This has a concave mirror with a radius of curvature of40 cm., giving a principal focus, therefore, at 20 cm. Thesight-hole is round and about 3^ mm. in diameter, cutthrough the glass ; this mirror can be tilted to an angle of25 degrees. As an improvement over such a mirror, and totake its place, the writer would recommend the mirror usedon his own ophthalmoscope, which has a radius of curvatureof 15 cm.; and the sight-hole, 21/ mm. in diameter, is notcut through the glass, but is made by rcmoxing the quick- «7 88 REFRACTION AND HOW TO REFRACT. silver. The glass at the siglit-hole gives additional reflect-ing surface, and at the same time does away with muchannoying aberration which results wlien the glass is BACK Fig. 79. The small sight-hole is an advantage, also, in lookinginto small puj)ils. The mirror, oblong in shape, 18 by 33mm., is secured at the center of its ends, b} two , to a hollow disc 4^^ cn- il diameter, in which is arevolving milled wheel, containing small spheres, eachabout 6 mm. in diameter. The series of spheres ranges OrilTIIALMOSCOPE. 89 from —I D, to —8 D., and irom +1 D. to -f 7 D. Thecentral aperture does not contain a lens, but is left open. When it is desirable to use any lens stronger than —8D. or +7 I^-, there is an additional quadrant, which can besuperimposed and turned into place at the sight-hole ; itcontains four lenses, — D. and —16 D., also + and ~{-16 D. With this quadrant and the spheres inthe milled wheel, any spheric combination can be madefrom zero to —24 D. or to +23 D. An index below thes
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