OPTICAL INSTRUMENTS, 



SPECTACLES. 



WHEN two lenses are mounted in a frame to fix before 

 the eyes, they are denominated spectacles ; the lenses are 

 employed to render the objects before the wearer more dis- 

 tinct. The eye, which consists of a convex lens, called the 

 crystalline lens, refracts the light proceeding from the object 

 placed before it in the same manner as a convex glass ; the 

 image of the object is formed at the focus of the lens, where 

 it is received on a screen at the back of the eye ; this 

 screen, called the retina, is an expansion of the optic nerve, 

 which conveys the sensation of vision to the mind. As the 

 crystalline lens of the eye will only produce distinct vision 

 when the focus is thrown on the retina, it is obvious that 

 should any defect occur with respect to that organ, indis- 

 tinct and imperfect vision will arise. Thus, if the lens of 

 the eye is not of a proper convexity to bring the image on 

 the screen, an indistinctness must ensue. This is the case 

 when the lens, through age, has become flattened ; the image 

 will then be thrown beyond the retina, and thus convey an 

 imperfect representation of the object to the mind. To 

 obviate this defect, we must make the rays pass through a 

 glass of sufficient convexity to assist the eye, and enable it 

 to form the image at the required place, which is in this 

 instance done by shortening the focal distance of the crystal- 

 line lens of the eye. If, on the contrary, the eye should be 

 too convex, or short-sighted, as is often the case with young 

 persons, then the image will not be formed at a sufficient 

 distance from the lens of the eye to reach the retina, and 

 thus imperfect vision of distant objects is produced. To 

 remedy this defect concave lenses must be resorted to, in 

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