LONG AND SHORT SIGHT. 283 



preserve eyes which can read small type at a moderate distance, 

 and can clearly define the outlines of distant objects. Nearly all 

 people, even if in their youth they possess good sight, lose it as 

 they grow older. They can discern distant objects well enough, 

 but, when they come to reading, they are obliged to hold the 

 book at arm's length before they can distinguish the letters. 



This defect is caused by the insufficient convexity of the lens, 

 so that the focus is thrown too far back, and it is corrected by 

 wearing spectacles sufficiently convex to supply the deficiency 

 in the lens of the eye. 



An admirable example of temporary long-sightedness is 

 familiar to every diver, though he may be unconscious of its 

 cause. Suppose that into very clear water of some twelve feet 

 in depth, a white object, say a common jam-pot, is thrown, it 

 can be clearly discerned from the shore, unaltered in shape or 

 size. But, when the diver searches for it, he sees at first only 

 something white, large, undefined, and wavering, and only finds 

 it resume its proportions as he approaches it. This phenomenon 

 is due to the pressure of the water upon the eyeball, which 

 flattens it, and so throws the focus too far back for a clear 

 image. Nowadays this defect is remedied by the use of very 

 convex spectacles, so convex, indeed, that, if worn in the air, 

 they would render the wearer incapable of seeing anything at 

 more than an inch or so away from him. But, when worn in 

 the water, they only supply the deficiency of the compressed 

 eyeball, and so restore the focus to its proper position. 



Those who suffer from short-sightedness can see with great 

 distinctness objects which are close at hand, but those at a 

 little distance seem to have no particular outline, and appear as 

 if they were viewed through a fog, thus causing a constant and 

 almost painful strain on the eyes. The cause of this defect is 

 the too great convexity of the lens, which therefore throws its 

 focus short of the required spot. The means of remedy are 

 exactly opposite to those which are used for long-sighted 

 persons, a concave lens being placed in front of the eye, so as 

 to throw the focus farther back, and relieve the organ from 

 the strain. 



Although we have not yet invented a machine that can alter 

 the focus at will, we may take a hint from Nature. We have 



