114 



HOW WE LIGHT OUR HOMES 



three common eye defects are nearsightedness , farsight- 

 edness, and astigmatism. 



Nearsightedness is caused by images of objects 

 falling in front of the retina when the focusing 



Nears ightedness 



Concave Lens- 



I 



\lff~ 



a corrected 

 FIG. 177. NEARSIGHTEDNESS AND HOW IT IS CORRECTED 



muscles are relaxed. Distant objects thus become in- 

 distinct. This defect is corrected by wearing eye- 

 glasses made of concave lenses (Fig. 177). These tend 

 to spread the light and thus throw the image back 

 upon the retina. 



Farsightedness is caused by the rays of light focus- 

 ing beyond the retina. In other words, the rays of 

 light strike the retina before an image has been 

 formed. Farsightedness is not so easy to detect as 

 nearsightedness because by muscular effort a far- 

 sighted person may be able to focus upon objects at 

 almost any distance. It must be detected through 

 indications of eye strain and fatigue rather than 



Farsightedness corrected 

 FIG. 178. FARSIGHTEDNESS AND HOW IT IS CORRECTED 



through indistinctness. Strained facial expression and 

 early fatigue are common symptoms of eyestrain, and 

 the cause must be determined and corrected by an 

 expert oculist. Farsightedness is corrected by convex 

 lenses which tend to converge the light rays more 

 and thus bring the image up to the retina (Fig. 178). 

 Astigmatism is caused by the lens or other parts 

 of the eye being irregular in curvature so that differ- 

 ent parts of the image are not in focus at the same 

 time. This produces a distorted and blurred image on 



the retina. This defect is especially annoying to stu- 

 dents and others who must do a great deal of reading 

 or looking at lines. It is one of the causes of headaches 

 and nervousness when not corrected. It can be reme- 

 died only by obtaining lenses ground to make up for 

 the error in the eye. 



If you find that you are unable to see clearly, or 

 that you suffer from headaches, nervousness, or pain 

 in the eyes, consult a reputable eye specialist at once. 

 The early correction of minor eye defects may prevent 

 serious trouble later. Always remember that seeing is 

 the most precious sense we have and that the eyes, 

 the organs of sight, are delicate, requiring great care 

 to keep them healthy. 



What are the parts of a camera and how does a 

 camera "take" pictures? A simple camera is like the 

 human eye in many respects. It is a light-tight box 

 with a convex lens at one end and a place for a photo- 

 graphic film or plate at the opposite end. When a pic- 

 ture is taken, reflected light from the object is allowed 



BBBB 



Courtesy Eastman Kodak Co. 



FIG. 179. FIRST FOLDING KODAK 



to pass through the lens for a short time, thus forming 

 an inverted image of the object on the film or plate 

 The surface of the film or plate usually contains a sil- 

 ver compound which is sensitive to light. Wherever 

 the light rays strike the silver compound they cause a 

 chemical action to take place. Where the light is 

 strongest the compound is changed the most and 

 where the light is weakest the compound is changed 

 the least. 



To regulate the size of the pencil of light that en- 

 ters the camera a diaphragm, or stop, is placed in front 

 of the lens. A large or small opening can be used. A 

 small opening increases the sharpness of the picture, 

 but the film or plate must be exposed for a longer 

 time. 



Cameras vary in cost from a dollar or two to hun- 

 dreds of dollars. The chief difference between a cheap 

 camera and an expensive one is in the lens. The ex- 



