SCHOOL HYGIENE 439 



Snellen's vision chart. Follow the directions that accompany 

 the cliart. 



DEMONSTRATION 35 



Purpose. To illustrate the working of the eye. 



Apparatus. Reading lens, cardboard, candle, concave lens, 

 convex lens. 



Directions. I. To show how the image is formed. Hold the 

 reading glass and the cardboard in line with the window till 

 images appear distinct on the cardboard. To what in the eye 

 do the cardboard and lens correspond ? How does the image 

 differ from the object? 



2. To illustrate the conditions for nearsightedness. Set a lighted 

 candle on a table and place a piece of white cardboard about 

 two feet away. Put the reading glass between the two in such 

 a way that an image of the candle appears on the cardboard. 

 To show nearsightedness, move the cardboard farther away 

 from the lens till the image is blurred. To show the remedy, 

 put a concave lens in front of the reading glass. 



3. To illustrate the conditions for farsightedness. Set up the 

 cardboard, candle, and reading glass as in the previous experi- 

 ment, till the image is clear. Then move the cardboard nearer 

 the lens till the image is blurred. This illustrates farsight. 

 To show the remedy, place a convex lens in front of the reading 

 glass. 



Defects of nose and throat. Defects of the nose and throat 

 are quite common. About two million children in the 

 United States are suffering from obstructed breathing. The 

 two most common defects are adenoids and enlarged tonsils. 

 These tend to obstruct the nasal passages and cause mouth 

 breathing. This is not as healthful as nose breathing be- 

 cause when air passes through the nose it is filtered of dust 

 and bacteria, warmed, and humidified. In some cases these 

 obstructions interfere seriously with the mental develop- 

 ment of the child. Inflammation of the nose and throat 

 often spreads to the ear through the eustachian tube and 



