346 APPLIED PHYSIOLOGY 



137. With a common magnifying glass show how a convex lens 

 brings rays of sunlight to a focus. Show also a photographer's camera. 

 Show the image which appears upon the ground glass. Then compare 

 the camera with the eye specimen, pointing out the resemblances. 



138. Have the students look steadily at a line of print and tell how 

 much they can read without moving their eyes. An inch and a half 

 will be all they can see at once. 



Next have them look steadily at an object, and notice how they can 

 see dimly all objects in a semicircle about them. 



139. Illustrate the duration of impressions by spinning a square top. 

 It will appear circular. 



140. Illustrate the exhaustion of the retina by having the students' 

 gaze at a square of black cloth upon a white paper. After a moment 

 let the students look steadily at the wall, when a square spot of light 

 will appear, for the part of the retina upon which the image of the 

 cloth fell is less exhausted than the rest, and so it sees the light from 

 the wall more clearly. 



141. Illustrate color blindness by taking a sheet of light pink paper. 

 Have the students first look steadily at a bright red object in a strong 

 light. Then gaze at the pink paper ; a green image of the first object 

 will appear, showing that a part of the retina has become exhausted for 

 the red rays, but can still see other colors. 



142. Notice the pupil of the eye and its varying size in different 

 lights. Shade another person's eyes with the hands, and, quickly 

 removing them, notice that the pupils grow smaller. Have a person 

 look steadily at your finger held a few feet in front of the eyes, and 

 then quickly bring the finger near the eye and notice that the pupil 

 contracts while looking at it near by. 



143. Illustrate a near-sighted eye by holding a magnifying glass in 

 front of the eye, which is the same as increasing the power of the lens. 

 Notice that the object must be brought nearer the eye. 



Show a double concave lens and explain that it scatters rays and so 

 is used in glasses for near sight. 



144. Illustrate far vision by looking through two magnifying glasses 

 of different strengths. Notice that the weaker glass must be held 

 farther away from the object. 



145. Place a book edgewise before the eyes and notice that one eye 

 sees one side and the other eye the other side. 



Now examine a stereopticon photograph of a statue, and notice that 



