. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, Fig. 1^0. the measurement of corneal curves by means of catoptricimages viewed through a telescope. The ophthalmometer consists of a telescope which con-tains a Wollaston birefrangent prism placed between two bi-convex lenses. Attached to the telescope is a graduated arc,upon which are placed two white enameled objects calledmires (targets). (See Figs. 132, 133, 134.) The left mireis stationary, and is made up of two 3 cm. squares, separated 148 REFRACTION AND HOW TO RE
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, Fig. 1^0. the measurement of corneal curves by means of catoptricimages viewed through a telescope. The ophthalmometer consists of a telescope which con-tains a Wollaston birefrangent prism placed between two bi-convex lenses. Attached to the telescope is a graduated arc,upon which are placed two white enameled objects calledmires (targets). (See Figs. 132, 133, 134.) The left mireis stationary, and is made up of two 3 cm. squares, separated 148 REFRACTION AND HOW TO REFRACT. by a black line 2 mm. wide ; the right mire is movable andgraduated into steps, each 5 mm. wide ; a black line passesthrough the middle of these steps. For purposes of focus-ing, the telescope is mounted on a movable tripod. Thepatient is seated with his chin and forehead resting in a. Fig. 131. frame. At the side of the frame, and attached to it, arc twoor four electric lights or Argand burners, which illuminatethe mires. The surgeon, looking through the eye-piece ofthe telescope, focuses the center of the patients corneauntil he sees two images of each mire cKarl\- ; tlun the two central imaiies for further studv and isjnores ASTIGMATISM. 149 the peripheral imaL^cs. The next step is to move theright-hand mire until these two images of the mires occupythe center or pole of the cornea, so that their inner edgesjust touch and the black line in each makes one continu-ous black line through both (see Fig. 133) ; and to do the
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