Page:The American Cyclopædia (1879) Volume XVI.djvu/411

VISION 391 age back to the retina, and one thus affected is said to be near-sighted ; this condition is myo- pia, and is remedied by concave eye glasses. Like any other lens, the crystalline has its optic centre ; this coincides nearly with its centre of form. The direction of every ray passing through this is nearly the same after as before transmission. Consequently, every such ray is an axis of a pencil of rays from some point of the object. The pencil of rays coming from the point directly before the eye has its axis corre- sponding with the principal or " optic axis " of the eye, passing through very nearly the cen- tres of the pupil and lens, and meeting the retina in a direction perpendicular to its sur- face, at the middle point of the small circular area which is the seat of most distinct vision. In providing for accurate vision under differ- ent circumstances, a very important element is the alternate contraction and dilatation of the pupil. When exposed to a strong light so intense that it would dazzle the eye and thus produce indistinctness of vision, the pupil con- tracts, shutting out a portion of the luminous beam, and thus reducing the amount of light to that which is readily borne by the eye. On the other hand, on first entering a darkened apartment with the pupils in a contracted con- dition, we may be incapable of perceiving sur- rounding objects; but within a few seconds the pupils enlarge, admitting a greater amount of light to the eye, and everything in the apart.- ment soon becomes distinctly perceptible. In some of the lower animals, whose vision adapts itself to extreme conditions of light and dark- ness, as in the cat, the variations in size of the pupil are exceedingly marked ; this opening being reduced in broad sunlight to a nearly imperceptible slit, while at night it is so ex- panded that the borders of the iris almost dis- appear. Notwithstanding the perfection of our visual apparatus, there is still only a small space directly in front of each eye within which objects can be distinctly seen. Outside of this space there is a field or circle of con- siderable extent within which we can perceive, though indistinctly, the presence of luminous objects; and this is called "the field of vis- ion. 1 ' In man, when using both eyes immov- ably fixed, the field of vision within its ex- treme limits subtends an angle of about 180 ; everything outside of this circle being invisi- ble to us, because the rays of light coming from the sides and behind cannot enter the pupil. In many of the lower animals, partic- ularly in birds and fishes, there is reason to believe that the field of vision is very much more extensive than this, owing to the lateral position of the two eyes, one being placed on each side of the head, and to the great promi- nence of the eyeball and its high refractive power, in consequence of which rays of light coming in an extremely oblique direction are bent inward and made to enter the pupil. It is certain, for example, that the ostrich can see objects directly behind him ; for a specta- tor placed in this position can see both the pupils, and is of course himself visible to the animal. In some fishes, if not in most of the class, there appears to be really no h'mit to the field of vision in any direction ; so that the animal can perceive rays of light coming from every point of the sphere, excepting that actually occupied by his own body. But even within the field of human vision there is only a single spot in its centre at which objects can be seen distinctly, that is, where their form and outlines are accurately perceived. Thus, when we stand in front of a row of upright stakes or poles, we can see those placed di- rectly in front of the eye with perfect distinct- ness, while those placed a little on either side, though perceptible, are indistinct. When w"e look steadily at the middle of a printed page we see the distinct form of each separate let- ter only immediately in front; above, below, and on each side of this spot, at successive dis- tances, the letters first become confused, then run into each other, and finally the distinction of letters and words is lost, and we perceive only the lines and spaces. The straight line extending directly in front of each eye, upon which alone objects are distinctly perceived, is called the "line of distinct vision." Every- thing above or below this line, to the right or the left, is perceived only in an imperfect man- ner. This is practically compensated for by the mobility of the eyeballs, by which we are enabled to direct the line of distinct vision to all parts of the space in front, in rapid succes- sion. When reading, for instance, the eyes fol- low the printed lines from left to right, seeing each letter and word distinctly in succession. Consequently an object can be distinctly per- ceived by both eyes simultaneously only when placed at a certain distance, namely, at that point where the two visual axes, or the lines of distinct vision for the two eyes, cross each other. As the eyes are situated two or three inches apart in their orbits, when they are both directed toward the same object their visual axes converge and meet at the situation of the object. Thus, although we look with two eyes, we see but a single object ; because, as the two lines of vision meet at a single point, the two distinct images exactly cover each other and so form but one. But either within or beyond this point vision becomes both double and indistinct. Thus, on holding up a slender rod at a distance of one or two feet in front of the face, and in the same range with any similar object, such as a door knob, on the opposite side of the room, if we direct both eyes to the rod we see it distinct and single, while the door knob appears as two indistinct images, one on each side. If we now direct the eyes to the door knob, it in turn becomes distinct and single, while the rod is doubled and confused in outline. This is because when we look at the nearer object so as to see it distinctly, the further one is still within the field of vision, and is therefore perceived ; but