INTRODUCTION TO OPTICS. 

 Fig. 36. 



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increase the divergence of the rays, the effect of a concave lens being 

 exactly the reverse of a convex one. By the assistance of such glasses, 

 therefore, the rays from a distant object fall on the pupil as divergent aa 

 those from a less distant object ; and, with short-sighted people, they 

 throw the image of a distant object back as far as the retina. Those who 

 suffer from the crystalline humour being too flat, apply an opposite 

 remedy : that is to say, a convex, lens LM {fig, 37), to make up for the 

 deficiency of convexity of the crystalline humour, O P. Thus elderly 

 people, the humours of whose eyes rre decayed by age, are under the 



Fig: 37. 



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necessity of using convex spectacles ; and when deprived of that resource, 

 they hold the object at a distance from their eyes, for the more distant the 

 object is from the crystalline, the nearer the image will be to it. These 

 two opposite defects are easily comprehended ; but it is difficult to con- 

 ceive how any sight can be perfect, for if the crystalline humour be of a 

 proper degree of convexity to bring the image of distant objects to a 

 focus on the retina, it will not represent near objects distinctly ; and if, on 

 the contrary, it be adapted to give a clear image of near objects, it will 

 produce a very imperfect one of distant objects. It is true, that every 

 person would be subject to one of these two defects, were it not in em- 

 power to increase or diminish, in some degree, the convexity of the crys- 

 talline humour, and to project it towards, or draw it back from the object, 

 as circumstances require. In a young, well-constructed eye, the fibres to 

 which the crystalline humour is attached have so perfect a command over 

 it, that the focus of the rays constantly falls on the retina, and an image 

 is formed equally distinct both of distant objects and of those which are 

 near. We cannot, however, see an object distinctly, if we bring it 

 very near to the eye, because the rays fall on the crystalline humour too 

 divergent to be refracted to a focus on the retina. The confusion, there- 

 fore, arising from viewing an object too near the eye, is similar to that 

 which proceeds from a flattened crystalline humour ; the rays reach the 

 retina before they are collected to v focus {fig. 38). If it were not for 

 this imperfection, we should be aMe to see and distinguish the parts of 



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