ISSN:0817-881X    

DOI:10.1111/j.1444-0938.1951.tb03233.x

出版商:Blackwell Publishing Ltd    

年代:1951

数据来源: WILEY

摘要:

SUMMARYThe balance of the mean spheres changes towards myopia in the eye which is observed from the nasal field and towards hyperopia in the eye which is observed from the temporal field.

ISSN:0817-881X    

DOI:10.1111/j.1444-0938.1951.tb03234.x

出版商:Blackwell Publishing Ltd    

年代:1951

数据来源: WILEY

摘要:

SUMMARYExact calculations based on trigonometrical ray‐tracing through the schematic eyes of Gullstrand and Tscherning reveal that oblique pencils diverging from the retina and passing through the centre of the pupil are afflicted by a considerable degree of radial astigmatism. The major portion of the aberration is shown to arise at the cornea, the contribution of the crystalline lens being relatively small. The bearing of these results on the practice of retinoscopy is briefly discussed.This investigation was undertaken at the suggestion of Mr. F. A. B. Hodd and is presented as an appendix to his paper. Its purpose was to see whether the schematic eye could throw any light on the effect of obliquity on the accuracy of retinoscopy. It is appreciated that the living eye presents several complex features which the schematic eye does not take into account. Nevertheless, the results obtained from calculations on the schematic eye may be taken as a first approximation.For tne sake ot simplicity the schematic eye is conceived as a system of co‐axial spherical surfaces, the media being homogeneous and the macula being situated on the optical axis. In the emmetropic schematic eye, a narrow pencil of rays diverging from the macula would emerge parallel to the eye's opticai axis (Fig. I).In retinoscopy as ordinarily practised, the optical axis of the observer's eye does not coincide with that of the subject, in which case the pencil of light used for observation is not a paraxial one. It diverges instead from a retinal point removed from the macula and traverses the optical system of the eye obliquely. It is therefore subject to the defect of radial astigmatism.The chief ray of a typical oblique pencil is shown in Fig. 2. It originates from an extra‐macular point Q and is directed towards the centre of the exit pupil, E', making an angle a, with the optical axis. After being refracted by the crystalline lens, the ray passes approximately through the centre of the actual pupil and finally emerges into air as though from the centre of the entrance pupil, E, making an angle a'k with the optical axis. In general, it can be predicted that the emergent pencil will be astigmatic, the vergence in the tangential meridian being greater than in the sagittal.The disposition of the tangential and sagittal planes is shown in Fig. 3. The outer circle represents the cornea, with its vertical and horizontal meridians intersecting at O. The smaller circles in dotted outline represent various positions of the emergent oblique pencil. The tangential meridian, TT, is invariably that containing the point O and the centre of the emergent pencil, the. sagittal meridian, SS, being always at right angles to the tangential.If the subject is instructed to look upwards (“just over my head”) the position of the emergent pencil used in retinoscopy may be as at (l), in which case the tangential meridian is vertical. On the other hand, if the patient is instructed to look sideways (“just past my ear”) the position may be as at (2), in which case the tangential meridian is horizontal. Assuming equal obliquity, the radial astigmatism at (2) will have the same magnitude as at (I), but its axis direction will he turned through 90°. In a position such as (3), the tangential and sagittal meridians wi

ISSN:0817-881X    

DOI:10.1111/j.1444-0938.1951.tb03235.x

出版商:Blackwell Publishing Ltd    

年代:1951

数据来源: WILEY

ISSN:0817-881X    

DOI:10.1111/j.1444-0938.1951.tb03236.x

出版商:Blackwell Publishing Ltd    

年代:1951

数据来源: WILEY