The extent of the field of vision for a single eye, the head being fixed, has been cal culated by Dr. Young. He found that the eyeball was capable of a movement of 55 degrees in every direction, so that a single eye may have perfect vision of any point within a range of 110 degrees.
We have not yet referred to the longitudinal range, or greatest distance of human vision; indeed, this range' varies so extremely that it is difficult to assign an arbitrary limit to it. Many uncivilized races, as the North American Indians, and the inhabitants of the vast Asiatic steppes, possess powers of sight which would appear almost incredi ble if they had not been thoroughly and frequently corroborated. Our information is more definite regarding the limits of human vision in regard to the minuteness of the objects of which it can take cognizance. Ehrenberg has carefully studied this subject; and has, arrived at the following results. The side of the smallest square magnitude usually visible to the naked eye—either of white particles on a black ground or con versely—is about of an inch; and with the greatest condensation of light and effort on the part of the observer, squares with a side as small as of an inch may be recognized, but without sharpness or certainty. Bodies smaller than these, when
observed singly, cannot be discerned by the naked eye, but may be seen when placed in a row. Much smaller particles may, however, be distinctly seen, if they powerfully reflect light; thus, gold-dust, which in none of its diameters exceeded of an inch, is easily discernible in common 'daylight. The delicacy of vision is far greater for lines than for minute areas, since opaque threads of of an inch may he discerned when held towards the light.
Various topics which the reader might perhaps have expected to find noticed, such, for instance, as " single vision with two eyes," "the appreciation of solid forms by the sense of vision," " correct vision with an inverted image on the retina," ejc., which belong fully as much to metaphysics as to physiology, are discussed in the article on Visroli. We may also refer those who desire information on these points to prof. Bain's treatise on The Senses and the Intellect.