. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, 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-hol
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, 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 thesi^jht-hole of the instrument records the strength of lens. Fig. 80. that may be in use ; minus lenses are usually marked in redand plus lenses in white. How to Use the Ophthalmoscope.—There are two wa)sor methods by which the ophthalmoscope may be used—the direct and the indirect. The Direct Method (see Fig. 80).—Proficicnc}- withthe ophthalmoscope docs not come except from long andconstant practice, and se\eral important matters shouldreceive very careful attention before the student attempts tostudy the interior of an e} 90 REFRACTION AND HOW JX) REFRACT. The Room.—This should be darkened by drawing theshades or closing the blinds ; the darker the room, the better. The Light.—This should be steady, clear, and bright ;a good lamp is suitable, but an Argand burner gives moreintense light, and is to be preferred, especially if it is placedon an extension bracket that can be raised or loweredand is capable of lateral movement. Position of Light and Patient.—The light should beseveral inches to one side and back of the patient, an
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