. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. od has been adopted. Optical mathe-maticians seek, namely, for a given system of refracting surfaces, certainfixed points, called cardinal points; and the knowledge of these is sufficientto enable us to construct and calculate the situation and size of the images ofgiven objects. The conditions are :—1st, that the system be centred, that is, 40 DIOPTRICS OF THE EYE. that the centres of curvature of all the refracting surfaces lie in a rightline, the axis of the system : 2ndly, tha
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. od has been adopted. Optical mathe-maticians seek, namely, for a given system of refracting surfaces, certainfixed points, called cardinal points; and the knowledge of these is sufficientto enable us to construct and calculate the situation and size of the images ofgiven objects. The conditions are :—1st, that the system be centred, that is, 40 DIOPTRICS OF THE EYE. that the centres of curvature of all the refracting surfaces lie in a rightline, the axis of the system : 2ndly, that the rays intersect the axis at onlysmall angles. The first condition appears to be amply fulfilled by thestructure of the eye, as the second is by direct vision, that is by lookingnearly in the direction of the line of vision. The situation and signification of the cardinal points are best understoodby studying them :—A in the case of only a single refracting surface ; B fora biconvex lens, with two refracting surfaces ; Cfor a combination of thesetwo into a compound system, such as the eye is. Fig. 91 A.—Refraction by a Spherical Surface. IY.—Cardinal Points. In Fig. 17, let k be the central point of the spherical surface h, on which, parallel to the axis A A, rays of light fall, a b and a b, coming from the medium with index n, and passing into the medium with index n. If n is > n, the parallel rays unite in the axis nearly in a point, the posterior focal point <p. The distance h <p = F, that is, the posterior focal distance is found n oby the known formula F = -—^— .... 1 a, n — ri wherein p the radius of the surface of curvatureis = h k. If on the same refracting surface, but in thedirection from A to A, rays fall, which in themedium with index n run parallel to the axis(they are in the figure represented by dots), thesealso unite nearly in a point in the axis, the an-terior focal point <p. The distance h <p = F\called the anterior focal
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