7IO THE SENSES 



astigmatism. The defect is demonstrated by numerous forms of astigmatic 

 charts, the most serviceable of which are the barred-letter test type, the clock 

 dial, or the dials shown in figure 463 or 483. Hang an astigmatic dial at a 

 distance of six meters and test the right and left eyes separately, as follows: 

 When the vision is focused on the center of the dial, if the eye is normal, 

 the three bars in each radius of the clock dial will be seen with equal distinct- 

 ness and have sharp black lines. In an astigmatic eye one or more of these 



FIG. 484. Diagram for Demonstrating the Blind Spot. 



radii will appear sharp and distinct, while the other will appear dim and 

 indistinct, the relative difference depending upon the degree of astigmatism. 

 Note the meridian of astigmatism in the right and left eyes separately. Use 

 the test set, and find the cylinder necessary to correct the astigmatism in 

 each eye and determine its meridian. 



Astigmatism is commonly shown by the presence of radii when one looks 

 at the stars at night, or by the ragged outline of a pin-hole in a card, when 

 held at arm's length against a white sky. In extreme cases outlines like 



FIG. 485. The Blind Spot with the Eye 30 cm. from the Paper. The irregularity of 

 outline is due to the larger blood vessels. 



the bars in the window sash or checks in clothing may be distorted, or some 

 of the lines may not even be seen. 



1 8. The Blind Spot. Look with the right eye at the spot in the ac- 

 companying figure at a distance of about 20 to 25 cm., covering the left eye. 

 Hold the spot in the line of direct vision and move the book to and from 

 the eye; in some cases it is necessary to rotate the book slightly. It will be 

 found that the cross to the right will, at a certain position, completely dis- 



