118 LABORATORY EXERCISES 



diameter and 3 inches apart. Hold the paper at arm's length, 

 shut the left eye, and fix the right eye upon the left spot. You 

 should see both spots. 



Now bring the paper toward the eye; when it is about 8 

 inches away, the right-hand spot will disappear, because its 

 image falls on the blind spot of the eye. At less than this dis- 

 tance the right-hand spot will reappear. 



b. On a piece of paper draw 2 circles each 3 inches in diam- 

 eter. Have the outlines heavy. On one of the circles draw 

 parallel vertical lines, all the same distance apart. On the 

 other circle draw parallel horizontal lines. Pifl the paper to 

 the wall, and look at it from some distance. Do the circles 

 appear to be true circles? How is each distorted? 



Repeat the experiment, using two squares. Results? 



If a wall paper contains many vertical stripes, what is the 

 effect upon the apparent height of the room? What is the 

 effect of horizontal stripes? 



If a person wishes to appear slender, ought he to wear cloth- 

 ing with vertical or with horizontal stripes? 



c. Out of a newspaper or an advertising circular cut several 

 letters % of an inch high. Paste them upon a sheet of white 

 paper, and pin the paper to the wall. Find out how far away 

 you can stand and yet see every letter distinctly. Measure this 

 distance and record it. 



Ask an elderly person to try this test; get the distance. Ask 

 a near-sighted person to do it. What are the results? . 



d. Get a sheet of light-pink paper (tissue paper), and hold 

 it before you. Now look steadily at a red circle in bright sun- 

 light; then look quickly at the pink paper. What kind of a 

 circle do you see on the paper? Tell why. 



e. If your school has a model of the eye, study it carefully, 

 and make out its parts. Compare them with Fig. 300, text. 



