. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, ches) and is 3 ^ cm. in diameteron a metal disc of the same size as theplane mirror. The sight-hole is simi-lar in size and made in the same wayas that of the plane mirror. The light should be steady, clear,and white, and secured to a movablebracket. For general use, the Argandburner is best. The Light-screen, or Cover-chim-ney.—For the purpose of interceptingthe heat this is made of thin asbestos,and the iris diaphragm attached to itregulates the amount of light desire
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, ches) and is 3 ^ cm. in diameteron a metal disc of the same size as theplane mirror. The sight-hole is simi-lar in size and made in the same wayas that of the plane mirror. The light should be steady, clear,and white, and secured to a movablebracket. For general use, the Argandburner is best. The Light-screen, or Cover-chim-ney.—For the purpose of interceptingthe heat this is made of thin asbestos,and the iris diaphragm attached to itregulates the amount of light desired.(See Fig. 140.) The room for retinoscopy should bedarkened and all sources of light exceptthe one in use should be excluded. Position of Light and Plane Mirror.—These may beas close together as 6 inches or as far apart as 6 is a matter of choice with the surgeon himself where heprefers to have them. The writer recommends, however,having the rays of light coming from the 10 mm. openingin the light-screen, at about 6 inches to the left and frontof the surgeon, so that the rays pass in front of the left eye. Fig. 140.—.^utliors IrisDiaphragm Chimney. 158 REFRACTION AND HOW TO REFRACT, and fall upon the mirror held before the right eye. Somesurgeons prefer having the light, with the 3 cm. opening inthe screen, placed over the patients head or to one side ofit. (Fig. 141.) TJic distance bctzvcoi the ligJit and mirrorwill not alter the direction of tJie rays of light which conicfrom the patients eye. Position of the Light and the Concave Mirror (, 142, 143).—As the purpose of the concave mirror inretinoscopy is to focus rays of light before they enter the
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