Visual Education (Jan-Nov 1920)

28 Visual Education photographic plate and the objective lens must be separated. The adjustment in the case of the eye is made otherwise in an extraordinary way. With the approach of the object the muscles of the eye contract in such a manner as to render tne lens more convex, and the consequence of this is that it is not necessary that the distance from the lens to the retina should be increased. Everyone has noticed that he must focus, or accommodate, Ms eyes according to the distance of the object at which he is looking. With advancing years, the muscles of the eye lose some of their elasticity, the power of accommodation diminishes, and glasses must be used for reading and seeing objects at short distances. The amount of light which passes through the lens L depends upon its size (is proportional to the square of its diameter) and the luminosity of the object. The more luminous the object the smaller the lens required to admit a given amount of light. In the case of the camera the aperture is controlled by means of the diaphragm. In the case of the eye, the aperture, or pupil, is automatically regulated by the iris. When the pupil is dilated' for seeing objects in comparative darkness, it admits more than ten times as much light as it does when it is contracted under the stimulus of strong sunlight. Moreover, the light which enters the eye may be still further reduced by squinting so as to interpose the eye lashes and to produce a shade by the eye brows. There is still another defect of lenses known as chromatic (color) aberration. A perfect lens does not bring parallel rays of different colors to a focus at the same point. Of the rays which are visible to the human eye, the violet and blue are brought to a focus nearest the lens and the red at the greatest distance. In the case of optical instruments, such as cameras and telescopes, chromatic aberration is largely overcome by suitable combinations of convex and concave lenses having different optical properties. In the case of eyes this defect is in no way remedied. This is the most important and about the only respect in which the optician's art is superior to nature's product. It is true that we are seldom conscious of this defect because it is always with us. Nearly everyone nas noticed that it is fatiguing to read blue and red letters mixed, or even to read blue letters on a red background. The reason is that the eyes are not in focus for both colors at once. We are accustomed to judge the distances of objects partly by the muscular effort required to focus upon them. As a consequence both of this habit and of the chromatic aberration to which the eye is subject, objects at a given distance whidi are red appear to be nearer than those which are violet or blue. It should not be inferred from the foregoing statements that the human eye is not a very remarkable optical instrument. It is so nearly perfect that if two points are at such a distance from the eye and from each other that their images on the retina are separated by as much as one twelve hundredth of an inch, they are seen as separate objects. On a portion of the retina having an area of only a fraction of a square inch, the details of a great landscape may be so accurately imaged that all of its numerous features may be clearly discerned. The direction of the eye is controlled and may be changed by six muscles which are attached to its exterior. One pair of muscles produces horizontal