= Solution
The <cornea> supplies much of the eye's converging <optical power>; the <crystalline lens> supplies additional, adjustable power through <ocular accommodation>. The transparent aqueous and vitreous media transmit the <light>. The <iris> controls the <pupil>, acting as an aperture rather than as the focusing element, and the <retina> is the receiving surface. The posterior <optic nerve> conveys the resulting neural signals; it does not carry an optical picture out of the eye.
Each sufficiently small object patch sends a pencil of rays through the <pupil>. Refraction at the corneal and lenticular surfaces brings that pencil towards a retinal image point. Pencils from different object heights meet at different retinal positions, producing a real inverted image. In a paraxial equivalent model,
$$
\frac1u+\frac1v=\frac1f,\qquad m=-\frac vu.
$$
The several physical refracting surfaces have been replaced here by an equivalent <thin lens>; this is a model, not a claim that all bending occurs at the <crystalline lens>. For a distant object, approximately parallel incoming pencils focus near the back focal surface. <Ocular accommodation> increases power for nearer objects while the retinal position remains nearly fixed.
\Image[/past-exam-of-the-medicine-course-of-the-university-of-oxford/1866/first-examination/physics-1866-eye-ray-geometry.png]
{title=Image formation by an equivalent eye and the two principal meridians of regular ocular astigmatism}
{description=Original paraxial schematics, not anatomical measurements. Left: one equivalent thin lens represents the combined corneal and lenticular power; parallel rays from one off-axis direction meet on a receiving plane. Right: ray fans in two perpendicular meridional sections have unequal focal distances. In three dimensions their focal images are perpendicular lines, not two complete point images.}
In <regular ocular astigmatism>, the two principal meridians are perpendicular but have different refractive powers, commonly because the <cornea> has different curvatures in those meridians. A point object then gives two line foci at different depths rather than a single sharp point. The difference is not merely an overall spherical focusing error. Refraction with differently oriented cylindrical trial lenses can locate the power and axis; <keratometry> or <corneal topography> measures corneal shape. <Cylindrical lens> or <toric lens> correction supplies the missing meridional power with the proper orientation. <Spectacles> and appropriately fitted <toric contact lenses> can provide such correction. A method for measuring corneal curvature should not be confused with a complete measurement of the eye's total refractive error.<Physiology of Astigmatism by the American Academy of Ophthalmology>[Regular astigmatism and principal meridians]{ref}<Astigmatism by the National Eye Institute>[Diagnosis and correction with glasses or contact lenses]{ref}
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