A STUDY OF THE SENSES 307 



ma-tism. Many people, on looking with each eye separately 

 at Fig. 141, find that some of the radiating lines stand out 

 sharply defined, while others are indistinct or blurred. In 

 reality, all the lines are equally distant from each other, and 

 the indistinctness referred to above is due to the fact that 

 the amount of curvature is not the same in all regions of the 

 cornea and crystalline lens. For this reason some of the rays 

 of light are not brought to a focus. Astigmatism, like near 

 and far sightedness, should be corrected by the use of proper 

 glasses, otherwise the constant eye strain is likely to cause 

 chronic headaches and other disorders of the body. 



Some people, too, are unable to distinguish clearly various 

 colors ; thus, red and green may appear the same to them. 

 In other words, such people are color blind. This cannot be 

 corrected by glasses, but can be to some extent by training. 



Hygiene of the Eyes. The eyes have, as we have seen, 

 wonderful powers of adapting themselves to varying con- 

 ditions. This adaptability often leads us to abuse them. 

 Thus, we frequently read when the light is insufficient, we 

 look steadily at objects until we suddenly find that we can- 

 not see clearly, and we read or study while riding in swiftly 

 moving trains. In these and other ways we compel our eyes 

 to make adjustments under trying conditions, and more or 

 less eye strain is sure to follow. 



When we read, we should make sure that the light is 

 sufficient, that it is steady, and that it comes over the left 

 shoulder. The type on the printed page should be little, 

 if any, smaller than that used in this book (ten-point type), 

 the lines should not be close together, and the paper should 

 not have a glossy surface to reflect the light into the eyes. 

 One should remember, too, that the eyes, like other organs 

 of the body, need frequent periods of rest. Hence study 

 hours should be followed by periods in which the eyes 

 are allowed to relax. Pupils who have defective eyesight 

 should make this known to the teacher, and should be 

 assigned the most favorable positions in the schoolroom. 



