This is the so-called Cartesian oval, which appears in a variety of contexts in visual optics from the prehistoric eyes of trilobites 2 to the study of human corneal topography. 20 Figure 3. Geometrical optics solution to spherical aberration of the reduced eye. Upper figure traces a ray from object to image space. The distance from S to paraxial focal point F is a measure of the spherical aberration of the model in image space.
Lower figure traces a ray from image to object space.
The distance from O to T is a measure of the spherical aberration of the model in object space. Figure 3 illustrates the geometrical optics solution to the problem of quantifying the degree of spherical aberration in the Indiana Eye. To find the distance A S we apply Snell's law to an incident ray parallel to the z-axis. AS may then be determined from the right triangle QAS as . Although these various quantities could be substituted into eqn.