Recent advances in ophthalmic science : The Boylston prize essay for 1865 . tion was also longago accepted as one of the ways in which distinct vis-ion is possible at different distances, and very strongif not conclusive aro;uments in its favor were adducedat the very beginning of this century by Thomas 5* 106 RECENT ADVANCES IN OPHTHALMIC SCIENCE. Young in a paper In the Philosophical Transactions;but it was reserved for two observers of our own day,Kramer and Helmholz, working independently of eacliother and nearly simultaneously, to reduce the prob-lem to an ocular and mathematical demonstr
Recent advances in ophthalmic science : The Boylston prize essay for 1865 . tion was also longago accepted as one of the ways in which distinct vis-ion is possible at different distances, and very strongif not conclusive aro;uments in its favor were adducedat the very beginning of this century by Thomas 5* 106 RECENT ADVANCES IN OPHTHALMIC SCIENCE. Young in a paper In the Philosophical Transactions;but it was reserved for two observers of our own day,Kramer and Helmholz, working independently of eacliother and nearly simultaneously, to reduce the prob-lem to an ocular and mathematical demonstration. Bythe ingenious instruments devised by these eminentobservers, the alterations of curvature in the crystal-line of an observed eye, as it is accommodated fornear or distant objects, are made perfectly obvious toan observer, while it is at the same time evident thatno change of curvature of the cornea takes place dur-ing the experiments. Fig. 16. Diagram explaining the change in position of the image formed by theanterior surface of the crystalline (after Bonders).. The observed eye is directed to the point A, and a candle andthe eye of the observer placed symmetrically on either side of A, THE FUNCTION OF ACCOMMODATION. 107 as shown in the figure. Now the only rays from the candlewhich can reach the eye of the observer are those which are re-flected from the central portions of the cornea, and anterior andposterior surfaces of the lens. The rays reflected from the cor-nea will then reach the observer as if they came from the pointa, and those from the posterior capsule as if they came from thedirection c. The rays from the anterior surface of the lens,when the eye is at rest, as is shown by the flatter outline ofthe lens, will reach the observers eye from the direction b. Im-ages of the flame, either real or virtual, will appear to the ob-server in the direction from which these rays respectively come,and will naturally be referred in position to the dark plane of thepup
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