The refractive anomaly alluded to is termed _astigmatism_ (from the Greek, α privative, στιγμα, a point--inability to see a point). The way in which objects appear to such a person will mainly result from the way in which he sees _a point_. Take, for example, the vertical to be the most, and the horizontal to be the least, refractive meridian: place a vertical line (Fig. 25, I) at a stated distance before the eye, and the line will appear elongated, owing to the diffusion image of each of the points composing it. It will also seem to be somewhat broadened, as at II. If the _vertical_ meridian is adapted to the distance of the vertical, the line will appear very diffuse and broadened, as at III. All these little diffusion lines overlap each other, and give the line an elongated appearance. Hence a straight line is seen distinctly by an astigmatic eye only when the meridian to which it is perpendicular is perfectly adapted to its distance. A _vertical line_ is seen distinctly when the _horizontal meridian_ is adapted to its distance. It appears _indistinct_ when its image is formed by the vertical meridian. The way in which an astigmatic person sees points and lines led to the discovery of this remarkable irregularity in the refraction of the eye. The late Astronomer Royal, Sir George Airy, suffered for some years until, indeed, he discovered how it could be corrected. This anomaly of curvature of the refractive surfaces of the eye is now known to prevail largely among the more civilised races of mankind. It is, then, of very great importance when using high powers of the microscope. In most persons the visual power of both eyes is rarely quite equal; on the other hand, the mind exerts an important influence, dominates, as it were, the eye in the interpretation of visual sensations and images. An example of this is presented in Wheatstone’s pseudoscope, known to produce precisely the opposite effect of his stereoscope--conveys, in fact, the _converse of relief_ produced by the latter and better known instrument. =Visual Judgment.=--The apparent size of an object is determined by the magnitude of the image formed on the retina, and this is inversely proportional to the distance. Thus the size of an image on the retina of an object two inches long at a distance of a foot, is equal to the image of an object four inches long at a distance of two feet. An object can be seen if the visual angle subtended by it is not less than sixty seconds. This is equivalent to an image on the _fovea centralis_ of the retina of about 4 µ[9] across, and which corresponds to the diameter of a cone: so that while we have had under consideration the optical and physical conditions of human vision, we have likewise taken a lesson on the action of lenses used in the construction of the microscope. The Theory of Microscopical Vision.