EYE 



2120 



EYE 



inside of them? We are so used to thinking 

 that they are outside that it is hard to realize 

 that what we see is not the world outside of 

 us, but the pictures of it on our retina formed 

 by the vibrations of light. A person who has 

 been blind all his life and suddenly' is given 

 sight is conscious that the picture he sees is 

 inside his head, and thinks all the objects in 

 it are close to him. He has to learn from 

 experience, as we did when we were small 

 children, to judge of such matters as distance 

 and motion. 



Why do the earth and sky seem to meet, 

 and the rails of a car track to come together in 

 the distance? If you hold this book about 

 five inches from your eye, the rays from the 

 top and the bottom of it will meet at right 

 angles on the cornea, and the image of the 

 book will occupy nearly the whole of the 

 retina. If you hold it farther away, as in the 



Explanation of illustration will be found in the 

 accompanying text. 



illustration, the rays from the top and bot- 

 tom will be closer to the rays from the center, 

 and if you had neither intelligence or experi- 

 ence you would say the book is smaller. Thus 

 as you look at a railroad track the rays from 

 rails near you enter the eye at an angle and 

 are refracted to the edges of the retina. But 

 the farther away a rail is, the closer are the 

 rays from it to those from the center of the 

 track. The rays from the very distant rails 

 occupy the same spot on the retina as the rays 

 from the center, and our eyes tell us that the 

 rails have met. 



Why the Wheel of a Moving Automobile 

 Appears Solid. The impression made on the 

 retina by each light wave lasts from one- 

 fiftieth to one-thirtieth of a second. If a 

 wheel is revolving rapidly, we get a new pic- 

 ture of each spoke before the old one has 

 disappeared; thus the image of the spoke is 

 in several places on our retina at the same 

 time. This is the principle upon which mov- 

 ing pictures depend, but in order to prevent 

 their being an indistinct blur, like the auto- 

 mobile spokes, each picture rests an instant 

 as it comes before the eye. 



Defects pf the Human Eye. Sight is made 

 up of a combination of three sub-senses, 

 namely, sense of light, sense of form and sense 



of color. The light sense enables us to appre- 

 ciate the varying intensities of light; the jorm 

 sense conveys information as to the shape of 

 an object; the color sense distinguishes the 

 color. All eyes are not normal from an opti- 

 cal or other standpoint. The great majority 

 present some defects, varying in degree and 

 character, which are commonly spoken of as 

 long- or short-sightedness, old sight, astigma- 

 tism, color blindness and cross-eyes. 



Care of the Eyes. We learn from a para- 

 graph above that the lens of the eye accom- 

 modates itself to our varying needs by chang- 

 ing its shape when we wish to look at an ob- 

 ject near at hand or one at a distance. Now 

 the nerves and muscles that cause the lens so 

 to adjust itself become very tired if over- 

 worked, and when badly abused we suffer 

 from what is called eye strain. The habit of 

 reading in a poor or fading light is a very 

 common cause of eye strain. Reading while 

 traveling on a jolting train is very injurious, 

 because the distance of the book from the 

 eyes is constantly changing, and the muscles 

 must work overtime to keep the lens in ad- 

 justment. A pernicious practice also is that 

 of reading while lying in bed or while ill. 

 Reading in these positions imposes great strain 

 upon the eyes, and if one be ill, in addition, 

 the general lowered muscle and nerve tone 

 associated with the illness is shared by the 

 eyes, and abuse of them at such times is dis- 

 tinctly injurious. Many serious diseases of 

 the eye result from the practice of permitting 

 untrained persons to attempt the removal of 

 dust, cinders, etc., from the eyes. This prac- 

 tice should be condemned vigorously. 



It would prove a profitable and wise cus- 

 tom if individuals contemplating entering a 

 profession or occupation requiring great de- 

 mands upon the eyes would in advance deter- 

 mine if their eyes are suited to the work about 

 to be assumed. Such forethought would often 

 spare the sad disappointments and hardships 

 which a later development of inadequate or 

 troublesome vision inflicts. 



The majority of eye diseases are caused by 

 germs which are communicated to the eye 

 through the medium of unclean hands, soiled 

 linens or other media. One cannot be too 

 careful about what touches this delicate and 

 priceless organ. Never dry the face with a 

 towel that is hung in a public wash room 

 the uncivilized "roller towel," for example for 

 it may harbor in its meshes countless germs of 

 diseases which may be communicated to the 



