Page:The New International Encyclopædia 1st ed. v. 20.djvu/226

* VISION. 178 VISION. that we are inwardly conseious of things which we have learned about from the outside. The Sm-llest Visible Retinal Images. In order to see two points distinctly (tw'o stars, for instance), it is necessary that the distance between them be such that tlie images fall upon two separate cones at the macula; hence this distance is governed by the size of the cones in this re- gion. If the two points are very close together so that their images fall upon a single cone, one impression onl}' is produced, and they will be seen as one. It has been calculated that an object must subtend an arc of at least 00 sec- onds in the visual field in order to be distinctly visible, making an image of 0.0045 of a milli- meter at the macula^ this representing the diameter of the cones in this area, and this esti- mate has been confirmed by experiment. See Ex- tension, Extent: Limen. The Rate of Ethereal Vibkations. As has al- ready been explained, the vibrations of the lumin- iferous ether constitute the normal stimulus of the retina. These ethereal vibrations dift'er in their rate of oscillation from 107,000,000,000,000 to 40,000,000.000,000.000 vibrations a second. The comparatively slow waves have the effect of warming bodies upon which they strike, and are known as heat waves; the most rapid ones have the property of breaking up chemical combina- tions and are known as chemical or actinic waves ; while those intermediate in their rate of oscillation cause changes in the rods and cones of the retina and are known as light waves. The light waves vavv in their rate of oscillation from 392,000,000,000,000 to 757.000.000.000,000 vibra- tions a second, and in wave lengths from 0.76G7 to O..3n70 micromillimeter. The intensity or brilliancy of light depends upon the amplitude of the vibrations. For color perception, see Visual Sensation. Binocular Vision. The two eyes ordinarily act together as a single instrument, and sight thus obtained is known as binocular vision. Each eye has its own field of vision ; a considerable part of the inner portion of each field overlaps the other, thus forming a binocular field of vision. Although we have two ej'es, we are conscious of only one visual impression ; it should be remem- bered that the fibres from the right half of each eye go to the visual area of the right hemisphere, from the left half to that of the left hemisphere. There being two retinal images for each object looked at, one for each eye, these two images must fall on corresponding parts of the retina in order that single binocular vision exist ; when this is the case, the two images are projected into space to the same spot and consequently coincide fir are suijcrimpospcl. To obtain single binoi'ular vision, the optical axes or visual lines of the two eyes must meet at the object seen directly; this is accomplished by the action of certain muscles of the eye. At the same time certain objects seen in indirect vision will also appear single — namely, those which lie within a jiar- tieular series of jioints known as the horopter. The external muscles of the eyeball serve to turn or rotate this organ in any direction ; one turns the eyeball inward and outward upon the Vertical axis; a second turns the eyeliall upward and downward ujion the horizontal axis; and a third set rotates the ej'eball upon the antero- posterior axis. By the associated action of two or more of the muscles of both eyes the gaze can be turned toward any object; this is done rapidly, unconsciously, and automatically. Thus, in read- ing, the internal muscle of the right eye acts with the internal muscle of the left ej'e and sufficient convergence results; when we turn the eyes to the left, the external muscle of the left eye acts with the internal muscle of the right eye, etc. As a consequence of paresis of any of the eye muscles, double vision (diplopia) results when- ever the object looked at is situated in a por- tion of the field corresponding to the sphere of action of the weakened muscle. If one eyeball be pressed upon with the finger so as to displace its visual line there will be double vision. In both of these instances the images of an object fall upon parts of the two retinte which do not correspond. In cases of cross-eyes (squint), the eye which is habituallj' turned in or out, and which is responsible for the second image, grad- ually learns to exclude the latter, so that the annoying double images are not perceived. Stereoscopic Vision. Binocular vision gives us a better estimation of solidity and distance than does vision with one e}"e. When both eyes are used, the degree of convergence of the visual lines and the amount of accommodation necessary for accurate focusing give us an estimate of the distance of the object. On account of the lateral separation of the two eyes and the resulting con- vergence of their visual lines, the image of each eye is slightly different; the image of the front of the object will be the same, l>ut in addition the right eye will see some of the right side of the object and the left eye some of its left side ; the mental combination of these images gives the impression of solidity or depth. This effect is produced, however, only within moderate dis- tances; when objects are remote, convergence of the visual lines and differences in the retinal images become inappreciable ; consequently dis- tant objects appear flattened and without relief. The stereoscope is an instrument used to look at two photographic pictures of the same object or scene from different points of view, one repre- senting what would be seen by the right eye, the other what would be observed with the left; when such a double photograph is looked at with this instrument, the two images are combined in the act of single binocular vision and pro- duce the effect of solidity or depth. See Dis- tance; Figure. The Duration of Visual Sensations. The visual sensation upon tlie retina persists after the luminous stinuilus has ceased; its duration is never less than one-tenth of a second, no matter how short the liniiinous stinuilus. The spokes of a rapidly revolving wheel appear as a .solid mass, because the images of the sjiokes upon the retina f<dlow one another so rapidly that one impression cannot fade away before another has replaced it. For the same reason, a rapidly re- vidving disk upon which all the colors of the spectrum are represented gives the effect of a white surface, a second color sensation being tlirown upon the retina liefore the impression of the fir-t has had time to disappear. This persistence of the retinal impression is responsible for positive after-images. If a brightly illuminatcil object be hmked at steadily, the impression upon the retina may be so per- sistent that the form and color of the object are