126 FIRST YEAR COURSE IN GENERAL SCIENCE 



137. Eye Glasses. Defects of vision, such as near- 

 sightedness, farsightedness, or unlike vision of the two eyes, 

 can generally be remedied by the use of spectacles or eye 



FIG. 61. A CAMERA 



o is a candle from which two rays of light are represented, a is the 

 opening in front of the lens of a camera, p is a plate or film, i is the image 

 upon the plate. Why is the image inverted? 



PUPIL- - 



FIG. 62. THE EYE 



In Fig. 62 the pupil is the opening through which light enters the 

 eye; the lens bends the rays of light and brings them together near the retina, 

 where the image is made. The muscles change the shape of the lens as the 

 distance of objects varies: other muscles make the pupil larger or smaller 

 according to the brightness of light. 1. Comparing Fig. 62 with the camera 

 (Fig. 61), tell why the image formed upon the retina is inverted? 2. Why 

 does it not seem so to us? 



glasses, which are often lenses, sometimes convex and some- 

 times concave. These, in combination with the lens of the 

 eye, bring the image in the right position with regard to 

 the retina. Without these external lenses, eye strain may 

 be caused by the effort of the muscles to change the shape 

 of the lens constantly. These muscles in the eye are 

 adapted only for occasional use, as when we look from a 



