. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, elmholtz).Allowing one millimeter for the thickness of the choroidand sclera, such an eye would have an anteroposteriormeasurement of about twenty-three millimeters. Parallelrays of light passing into such an eye in a state of restwould focus on the macula. Cardinal Points (Fig. 57).—Images formed upon theretina are the result of refraction by three refracting sur-faces and three refracting media. The refracting surfaces 58 CARDINAL POINTS. ^g are the anterior surface o
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, elmholtz).Allowing one millimeter for the thickness of the choroidand sclera, such an eye would have an anteroposteriormeasurement of about twenty-three millimeters. Parallelrays of light passing into such an eye in a state of restwould focus on the macula. Cardinal Points (Fig. 57).—Images formed upon theretina are the result of refraction by three refracting sur-faces and three refracting media. The refracting surfaces 58 CARDINAL POINTS. ^g are the anterior surface of the cornea and the anterior andposterior surfaces of the crystalline lens. The refractin*-media are the cornea (and aqueous humor forming a convexlens), the crystalline lens, and the vitreous humor. Theserefracting surfaces and media represent a compound dioptricsystem, centered upon the optic or principal axis—/. c, aline drawn from the pole of the cornea to a point betweenthe nerve and fovea. On the principal axis, therefore, are situated the anteriorand posterior principal foci, the anterior and posterior nodal. Fig. 57. points, and the anterior and posterior principal points Theanterior principal focus is situated upon the optic +mi- ill front of the corneal apex. The pos-terior principal focus is situated )-mm. back of theposterior surface of the lens. The nodal points are situ-ated about 7 mm. back of the cornea, and correspondapproximately to the optic center of this compound re-fracting system ; and as they are so close together, they areconsidered as one for all purposes in the study of the for-mation of images. The first or anterior principal point is 6o REFRACTION AND HOW TO REFRACT. situiited mm. back of the anterior corneal surface, andthe second or posterior principal point is situated the anterior surface of the cornea. The principalpoints are so closely situated that they are considered asone. The anterior focal distance
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