Text-book of ophthalmology . the exactest portionof ophthalmology, and, in fact, of all medicine, for it is based directlyupon the application of physical and mathematical laws to the eye. Theselaws, therefore, as far as they will require consideration here, must be sup-posed to be known in advance. ANOMALIES OF REFRACTION AND ACCOMMODATION CHAPTER ITHE THEORY OF GLASSES 741. Convex and Concave Glasses.—The refractive power of a lens isdetermined by the position of its principal focus. Under the latter nameis denoted that point at which rays are united which come from an infinitedistance, and
Text-book of ophthalmology . the exactest portionof ophthalmology, and, in fact, of all medicine, for it is based directlyupon the application of physical and mathematical laws to the eye. Theselaws, therefore, as far as they will require consideration here, must be sup-posed to be known in advance. ANOMALIES OF REFRACTION AND ACCOMMODATION CHAPTER ITHE THEORY OF GLASSES 741. Convex and Concave Glasses.—The refractive power of a lens isdetermined by the position of its principal focus. Under the latter nameis denoted that point at which rays are united which come from an infinitedistance, and hence are parallel when they strike the lens. In convex lenses which render parallel rays convergent, the principalfocus (F, Fig. 352) lies on the opposite side to that upon which the rays all the parallel rays are collected or condensed (hence condensinglens). If the rays emanate from a point, they will also be united into a singlepoint,but if the object that emits the rays has an extension in space, they are L. Fig. 352.—Union of Parallel Rays, refracted by a Convex Lens. I D-^ Fig. 353.—Dispersion of Parallel Rays by a Concave Lens. united into a diminished,inverted image of the object. This image is real—i. e., formed by an actual union of the rays at this spot. Just as rays fallingupon the lens in a parallel direction ultimately reach the principal focus, F, soalso rays which go in the opposite direction, from F, and impinge upon thelens, will emerge from the latter in a parallel direction. Concave lenses so refract the rays which impinge upon them in a par-allel direction that the latter become divergent when they emerge; hencethe name dispersing lenses (Fig. 353). These rays never come together,but, on the contrary, diverge farther and farther from each other. Hencean actual (real) focus—i. e., point of union of the rays—does not exist inthis case. But if an observer is stationed behind the lens—e. g., at a—andreceives the diverging rays upon hi
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