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If the person we have been speaking of were to wear spectacles with concave glasses of 20 inches focus, he would be able to see distinctly any distant object. It does not, however, appear that they would not inconvenience him when he wished to look at a near object, one at the distance of 10 inches, for example.
By putting 10 for ∆, and 20 for F in the common formula,
1∆″=1F+1∆;
we find
1∆″=120+110=320=1623.
If therefore this person can see distinctly without a glass, an object at about 6 inches from his eye, he will find no inconvenience from his spectacles, provided he does not use them to look at any thing nearer than 10 inches.
141. Let us see now how we can remedy the defect of vision in an aged person, who cannot discern an object within 24 inches from his eye.
A double-convex lens of 24 inches focus will make such an object appear infinitely distant; others within 24 inches will be removed to distances more or less considerable; the nearest object distinctly discernible will be that having its image at 24 inches from the eye. Putting 24 for ∆″, and −24 for F, we find
124=−124+1∆; ∴ ∆=12.
From this we collect, that spectacles with convex glasses of 24 inches focus may be used for any object from 24 to 12 inches from the eye. Within the lesser of these distances the lenses are inefficient, and beyond the other, the image would be more than infinitely distant, that is, the rays entering the eye would be made convergent, and consequently unfit for vision.
142. There is a disease of the eye called the Cataract, in which the chrystalline lens losing its transparency, the patient becomes blind. The only cure known is to extract that part of the
