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uses a combination of two or more lenses of diflFerent kinds of glass. 

 In the eye no such compensation exists, and, in this respect, the 

 instrument maker has perhaps improved on nature's handiwork. 

 It was mainly, in respect of this defect, that Helmholtz made his 

 well knoAvn remark, that if a workman had sent him such an 

 instrument as the eye, he should have returned it to him for a 

 few alterations and improvements. Under certain conditions we 

 do see objects with coloured margins, but the inconvenience from 

 this cause is so slight, that the fact itself has only comparatively 

 recently been discovered. These defects and the methods of 

 removing them, were illustrated by experiments. 



To obtain an image in accurate focus on the glass screen of the 

 camera, the photographer must alter the distance of the lens from 

 the screen for every change in the position of the object. It is 

 obvious that no such motion is possible with the lens of the eye, but 

 a far more beautiful mechanism exists by which the range of the 

 lens is altered automatically, so that the image of the object how- 

 ever distant, is always focussed on the retina. After many years' 

 discussion it is now accepted as a fact, that the shape of the lens is 

 altered by ligaments attached to certain muscles, so that the lens is 

 compressed and rounded when the eye is looking at near objects. 

 By age the lens becomes more dense and loses this power of 

 "accommodation," until finally only distant objects can be seen 

 distinctly, and the person is said to be "long sighted." 



At 21 years this normal sight has a range from 4 Jin. to an 

 infinite distance, while at 40 the "near point" has increased to 9in. 

 But for a sustained effort of vision the object should, if possible 

 be always at twice this distance. The causes of short and long 

 sight were explained and illustrated by experiment, and the very 

 great injury and danger to the eye often caused by the disinclination 

 to use spectacles were pointed out. 



It is a common error to suppose that short sight improves with 

 age, the fact being that short-sighted people like aU others, lose the 

 power of looking at near objects, and as they grow old do not gain 

 anything at any time. Certain other small defects in vision were 

 next referred to : these were " astigmatism," or the inability to 

 distinguish, with equal clearness at the same time, horizontal and 



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