An eye is adapted to the distance of the luminous point when the circle of diffusion, received upon the retina, is at its minimum. This is the case when the sentient layer of the retina lies between the two foci E. In this case the point will appear as a small circle, composed of the two colours, that is to say--violet. If the retina be _in front_ of this point, at the focus of the _blue_ rays for instance, the eye will perceive a _blue point surrounded by a red circle_, the latter being formed by the periphery of the luminous cone of red rays, which are focussed only after having passed the retina. The blue point will become a circle of diffusion larger in proportion as the retina is nearer the dioptric system, or as the focus for blue is farther behind it. But the blue circle will always be surrounded by a red ring. If, on the contrary, the retina is _behind the focus for red_, the blue cone will be greater in diameter than the red, and we shall have a _red circle of diffusion_, larger in proportion as the retina is farther from the focus, but always _surrounded by a blue ring_ M. If the blue-red point is five metres, or more, distant, the _emmetropic_[7] eye will evidently see it more distinctly, _i.e._, as a small _violet point_; the _hyperopic_ eye, whose retina is situated _in front_ of the focus of its dioptric system, will see a _blue circle, surrounded by red_; the _myopic_ eye, whose retina is _behind_ its focus, will see a _red circle, surrounded by blue_. The size of these circles will be either larger or smaller when the principal focus of the eye is either in _front_ of or _behind the retina_.[8] The refractive surfaces of a perfectly formed eye are very like an ellipsoid of revolution with two axes, one of which, the major axis of the ellipse, is at the same time the optic axis and that of rotation; the other is perpendicular to it, and is equal in all meridians. Eyes, however, perfectly constructed are rarely met with. The curvature of the cornea is nearly always greater in one meridian than in another. Its surfaces then cannot be regarded as entirely belonging to an ellipsoid of revolution, since the solid figure, of which the former would constitute a part, has not only two axes, but three, and these unequal. This irregularity is not, however, always great enough to produce discomfort and it is therefore disregarded. But in other cases the difference of curvature in the different meridians of the eye attain to a higher degree, and vision falls far below the average. [Illustration: Fig. 25.--Lines as seen by the Astigmatic.]