346 



GENERAL SCIENCE 



moment's time for the eye to adjust itself so as to form a sharp 

 image on the retina. 



FIG. 225. A convex lens brings the light rays together. 



Near and Far Sightedness. If the eyeball is flattened the person 

 is said to be far sighted or to be suffering from hypermetropia. If 

 the eyeball is elongated the person is near sighted or suffering from 



myopia. Artificial lenses must be placed 

 in front of the eye in order to relieve 

 the muscles from severe tension, and 

 to bring the rays of light to a proper 

 focus without undue effort on the part 

 of the muscles. 



Snellen Test. Hold the test sheet 

 twenty feet away from the person. 

 Be sure the light illuminates it but 

 does not shine in the eyes. Place the 

 sheet on a level with the eyes. Test 

 each eye separately by holding a piece 

 of black paper over one eye. If the 

 person wears glasses, make the test 

 with the glasses on. If a person can 

 read the lines marked fifteen feet, at 

 a distance of twenty feet from the 



chart, that person is far sighted. If a person cannot read the 

 line marked twenty feet, he is near sighted, and the seriousness 

 of the trouble may be determined by the marking of the line which 

 the person is able to read. Unless there is a sign of eye strain, 



FIGS. 226 and 227. The focus- 

 ing of parallel and divergent 

 rays in near sightedness. The 

 correction of myopia by 

 means of a concave lens. 



