SPECTACLES. 81 



LESSON 37. 



Optical Instruments. 



Land'scape, the prospect of a country, — also a picture represent- 

 ing an extent of space with the various objects on it. 



Glob'ule, a small particle of matter of a globular or spherical 

 figure. 



As the sight is the most noble and extensive of all our 

 senses; as we make the most frequent use of our eyes in 

 all the actions and concerns of life ; that instrument which 

 relieves the eyes when decayed, and supplies their defects, 

 must be estimated as one of the greatest of advantages. 

 Sight may be defective in various ways. Some eyes are too 

 flat, others are too convex or round ; in some, the humours 

 lose a part of their transparency, and on that account, much 

 of the light that enters the eye is stopped and lost in the 

 passage, and every object appears dim. Spectacles are in- 

 tended to collect the light and to bring it to a proper degree 

 of convergency. The honour of their invention was claimed 

 by Salvinus Armatus, a nobleman of Florence, who died 

 in 1317, and the fact was inscribed on his tomb. When the 

 eye is too flat, the rays proceeding from objects do not con- 

 verge to a focus so soon as they reach the retina ; in this 

 case a convex glass is necessary, for it has the property of 

 converging the rays, and of course, when suited to the eye, 

 ef bringing them to a focus, and forming an image on the 

 retina. When the eye is too convex, the rays of light are 

 converged to a focus before they reach the retina ; to remedy 

 this, a concave glass is used, which causes the rays to di- 

 verge, and prevents their coming to a focus too soon. Short- 

 sighted persons bring objects close to their eyes ; it has a 

 similar effect to that produced by concave glasses ; for the 

 nearer an object is brought to the eye, the greater is the 

 angle under which it is seen, that is, the extreme rays, and 

 of course all the others, are made more divergent. But per- 

 sons whose eyes are too flat, when examining an object, hold 

 it at a distance, for the farther an object is held from their 

 eyes, the less is the divergency of its rays, that is, the smaller 

 is the angle under which it is seen : the focal distance is in- 

 creased, and an image is properly formed on the retina. In con- 

 sidering vision as achieved by the means of an image formed 



