COMMON THINGS SPECTACLES. 



their utility is ubiquitous. In the palace of the monarch and in 

 the cottage of the peasant their beneficent influence is equally 

 diffused. It is remarkable also, that, unlike most other produc- 

 tions of art and science, cost can add nothing to their perfection. 

 Those of the millionaire may be mounted in gold, and those of the 

 humble labourer in iron ; but the optical medium, the glass lenses 

 to which they owe their perfection, must be the same. 



2. It is good that an object of such unbounded usefulness 

 should be generally understood. The more completely and 

 clearly the principles on which the application of the instru- 

 ment depends are comprehended, the greater will be the extent 

 of the benefit which each individual will derive from them, and the 

 less frequent will be the inconveniences and evils resulting from 

 their abuse. 



Before it is possible, however, to comprehend the principle and 

 the right use of spectacles, it is indispensably necessary to be 

 acquainted with the structure and functions of the eyes, and such 

 readers as have not already obtained that preliminary knowledge 

 are referred for it to our tract on that subject. 



The defects incidental to the sense of sight have been briefly 

 noticed in that tract, and the optical expedients by which 

 remedies have been obtained for them have been stated. We 

 propose here to resume that subject, and to present other and 

 more developed illustrations of it. 



3. When an object is placed at a certain distance from the eye, 

 a small picture or image, as it is called, of the object is produced 

 upon the posterior surface of the coating which lines the inside of 

 the spherical shell, called the eye-ball. This coating, upon which 

 the picture is thus formed, is called the retina. 



It is this picture on the retina which enables us to see the object. 

 If this picture be obscure, falsely coloured, confused, or indistinct, 

 our vision of the object will also be obscure, falsely coloured, con- 

 fused, or indistinct. 



In its natural and healthy state the structure of the eye is such 

 that the pictures of all objects presented to it thus formed upon 

 the retina, are clear, rightly coloured, and perfectly distinct in 

 form and outline. In individual cases, however, eyes are vari - 

 ously defective. 



4. If the coats and humours, through which the rays of light, 

 proceeding from external objects ought to pass, be not in any 

 degree transparent, no picture whatever is formed on the retina, 

 and the subject is blind. 



5. If the coats and humours are imperfectly transparent the 

 picture will be obscure, being formed only by the rays of light 

 partially transmitted through the humours. 



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