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. 3. The degree of vivacity in the colours and tints of objects. 4. The degree of distinctness in the outline and minute parts of objects. 5. To these criteria we may add the sensation of muscular action, or rather effort, by which we close the pupil in accommodating the eye to near distances, and produce the accommodation. With all these means of estimating distances, it is only by binocular vision, in which we converge the optical axes upon the object, that we have the power of _seeing distance_ within a limited range. But this is the only point in which Monocular is _inferior_ to Binocular vision. In the following respects it is _superior_ to it. 1. When we look at oil paintings, the varnish on their surface reflects to each eye the light which falls upon it from certain parts of the room. By closing one eye we shut out the quantity of reflected light which enters it. Pictures should always be viewed by the eye farthest from windows or lights in the apartment, as light diminishes the sensibility of the eye to the red rays. 2. When we view a picture with both eyes, we discover, from the convergency of the optic axes, that the picture is on a plane surface, every part of which is nearly equidistant from us. But when we shut one eye, we do not make this discovery; and therefore the effect with which the artist gives relief to the painting exercises its whole effect in deceiving us, and hence, in monocular vision, the _relievo_ of the painting is much more complete. This influence over our judgment is beautifully shewn in viewing, with one eye, photographs either of persons, or landscapes, or solid objects. After a little practice, the illusion is very perfect, and is aided by the correct geometrical perspective and _chiaroscuro_ of the Daguerreotype or Talbotype. To this effect we may give the name of _Monocular Relief_, which, as we shall see, is necessarily inferior to _Binocular Relief_, when produced by the stereoscope. 3. As it very frequently happens that one eye has not exactly the same focal length as the other, and that, when it has, the vision by one eye is less perfect than that by the other, the picture formed by uniting a perfect with a less perfect picture, or with one of a different size, must be more imperfect than the single picture formed by one eye. CHAPTER III. ON BINOCULAR VISION, OR VISION WITH TWO EYES.