. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, feet, andl)rcfcrably under a cycloplegic. The method ot the tost isto mo\c one flame along the metal arm until the two lightsappear to The scale, as marked on the arm, givesthe approximate strength of lens necessar) to correct theametropia. By raising or lowering the arm an) meridian ASTIGMATISM. H7 may be tested. It is a most ingenious test, but not incommon use. 12. The Ophthalmometer (.see Figs. 131 and 132).—This name literally means an e)e measure, but as ti
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, feet, andl)rcfcrably under a cycloplegic. The method ot the tost isto mo\c one flame along the metal arm until the two lightsappear to The scale, as marked on the arm, givesthe approximate strength of lens necessar) to correct theametropia. By raising or lowering the arm an) meridian ASTIGMATISM. H7 may be tested. It is a most ingenious test, but not incommon use. 12. The Ophthalmometer (.see Figs. 131 and 132).—This name literally means an e)e measure, but as tiie in-strument only measures the different radii of corneal curva-ture, a much better name would be keratometer, or measureof the corneal radii. The object of the ophthalmometer is. 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
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