ADDITIONAL TOPICS IN HUMAN BIOLOGY 165 



Figure 54, find that some of the radiating lines stand out sharply de- 

 fined, 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 some of the rays of light are not 

 brought to a focus. Astigmatism, like nearsightedness and far- 

 sightedness, should be corrected by the use of proper glasses, other- 

 wise constant eyestrain is likely to 

 cause headaches and other disorders 

 of the body. 



Some people, too, are unable to dis- 

 tinguish clearly various colors; thus, 

 red and green may appear the same to 

 them. In other words, such people 

 are color blind. Color blindness can- 

 not be corrected by glasses, but may 

 be to some extent by training. 



rp,, FIG. 54. Test for astigmatism. 

 248. Hygiene of the eyes. The 



eyes have, as we know, wonderful powers of adapting themselves to 

 varying conditions. 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 cannot 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 eyestrain 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 in which 

 most of this book is printed, 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 at once secure 

 proper glasses. 



