The stereoscope

by David Brewster

Fig. 5, A.

Fig. 5, B.

But though we are ignorant of the manner in which the mind takes cognizance through the brain of the images on the retina, and may probably never know it, we can determine experimentally the laws by which we obtain, through their images on the retina, a knowledge of the direction, the position, and the form of external objects.

If the eye mn consisted only of a hollow ball with a small aperture h, an inverted image, ab, of any external object ab would be formed on the retina ror, exactly as in [Fig. 4]. A ray of light from a passing through h would strike the retina at a, and one from b would strike the retina at b. If the hole h is very small the inverted image ab would be very distinct, but very obscure. If the hole were the size of the pupil the image would be sufficiently luminous, but very indistinct. To remedy this the crystalline lens is placed behind the pupil, and gives distinctness to the image ab formed in its focus. The image, however, still remains inverted, a ray from the upper part a of the object necessarily falling on the lower part a of the retina, and a ray from the lower part b of the object upon the upper part b of the retina. Now, it has been proved by accurate experiments that in whatever direction a ray aha falls upon the retina, it gives us the vision of the point a from which it proceeds, or causes us to see that point, in a direction perpendicular to the retina at a, the point on which it falls. It has also been proved that the human eye is nearly spherical, and that a line drawn perpendicular to the retina from any point a of the image ab will very nearly pass through the corresponding point a of the object ab,[31] so that the point a is, in virtue of this law, which is called the Law of visible direction, seen in nearly its true direction.

When we look at any object, ab, for example, we see only one point of it distinctly. In [Fig. 5] the point d only is seen distinctly, and every point from d to a, and from d to b, less distinctly. The point of distinct vision on the retina is at d, corresponding with the point d of the object which is seen distinctly. This point d is the centre of the retina at the extremity of the line aha, called the optical axis of the eye, passing through the centre of the lens lh, and the centre of the pupil. The point of distinct vision d corresponds with a small hole in the retina called the Foramen centrale, or central hole, from its being in the centre of the membrane. When we wish to see the points a and b, or any other point of the object, we turn the eye upon them, so that their image may fall upon the central point d. This is done so easily and quickly that every point of an object is seen distinctly in an instant, and we obtain the most perfect knowledge of its form, colour, and direction. The law of distinct vision may be thus expressed. Vision is most distinct when it is performed by the central point of the retina, and the distinctness decreases with the distance from the central point. It is a curious fact, however, that the most distinct point d is the least sensitive to light, and that the sensitiveness increases with the distance from that point. This is proved by the remarkable fact, that when an astronomer cannot see a very minute star by looking at it directly along the optical axis dd, he can see it by looking away from it, and bringing its image upon a more sensitive part of the retina.

But though we see with one eye the direction in which any object or point of an object is situated, we do not see its position, or the distance from the eye at which it is placed. If a small luminous point or flame is put into a dark room by another person, we cannot with one eye form anything like a correct estimate of its distance. Even in good light we cannot with one eye snuff a candle, or pour wine into a small glass at arm’s length. In monocular vision, we learn from experience to estimate all distances, but particularly great ones, by various means, which are called the criteria of distance; but it is only with both eyes that we can estimate with anything like accuracy the distance of objects not far from us.

The criteria of distance, by which we are enabled with one eye to form an approximate estimate of the distance of objects are five in number.

1. The interposition of numerous objects between the eye and the object whose distance we are appreciating. A distance at sea appears much shorter than the same distance on land, marked with houses, trees, and other objects; and for the same reason, the sun and moon appear more distant when rising or setting on the horizon of a flat country, than when in the zenith, or at great altitudes.

2. The variation in the apparent magnitude of known objects, such as man, animals, trees, doors and windows of houses. If one of two men, placed at different distances from us, appears only half the size of the other, we cannot be far wrong in believing that the smallest in appearance is at twice the distance of the other. It is possible that the one may be a dwarf, and the other of gigantic stature, in which case our judgment would be erroneous, but even in this case other criteria might enable us to correct it.