Recent advances in ophthalmic science : The Boylston prize essay for 1865 . d to a distinct retinal image when the eye is atrest, and even parallel rays from remote objects requirean effort of accommodation, we can understand thepain and fatigue resulting from long-continued use fornear objects, from which divergent rays only are re-ceived. At first, however, these objects are -seen 142 RECENT ADVANCES IN OPHTHALMIC SCIENCE. clearly, for the eye is capable, fro tempore^ of therequisite accommodative effort; but this strain of theciliary muscle cannot be long kept up without produ-cing a sensat
Recent advances in ophthalmic science : The Boylston prize essay for 1865 . d to a distinct retinal image when the eye is atrest, and even parallel rays from remote objects requirean effort of accommodation, we can understand thepain and fatigue resulting from long-continued use fornear objects, from which divergent rays only are re-ceived. At first, however, these objects are -seen 142 RECENT ADVANCES IN OPHTHALMIC SCIENCE. clearly, for the eye is capable, fro tempore^ of therequisite accommodative effort; but this strain of theciliary muscle cannot be long kept up without produ-cing a sensation of lassitude and discomfort, whichpresently extends from the eye to the brow, is fol-lowed perhaps by a loss of clearness of perception, andat length compels a cessation of the occupation whichrequires such tension. The eyes being left for a whilein repose recover their adaptive power, and can beagain used for a time, to become again disabled by areciuTence of the symptoms after a longer or shorterperiod. Fig. 27. Hypermetropia coiTccteii by the use of a convex Figure 27 represents a liypermetropic eye at rest, in whichcondition the convergent rays A A are refracted to a focus uponits retina N. Parallel rays A A being, under the same condi-tions, refracted to a focus at the normal position of the retina the interposition of the convex lens L, the parallel rays A Aare rendered convergent, and thus enter the eye in such a con-dition as permits the formation of a distinct image upon the ac-tual retina N. As in myopia, a peculiar form of the eyeball may HYPERMETROPIA. 143 be observed on inspection. The patient being told tolook strongly inwards, we find at the outer canthus anevident shortness of the globe from before does not fill the space at the outer angle like a my-opic, or even a normal eye, but its posterior surfaceshows a rapidly retreating curve. Its antero-posteriordiameter has perhaps only four fifths of the length ofthe normal standard. In ex
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