714 THE SENSES 



five degrees. When the letters marked lt twenty feet" or "six meters" 

 represent the limit of accurate identification, the visual acuity is said to 

 be i, or normal. If the line marked "thirty feet" is the limit, then the 

 acuity is one and a half; if "fifteen feet," then the visual acuity is three- 

 fourths, etc. 



If the eyes tested are astigmatic, or have other optical defects, these 

 must first be corrected before testing for visual acuity. 



29. The Test Set. The student is recommended to close the experi- 

 ments on the eye by fitting glasses for himself and at least two others. He 

 should correct for the defects that have been revealed in the preceding experi- 

 ments, especially for astigmatism; myopia, or hypermetropia; and presby- 

 opia. Of course each eye must be tested and fitted separately. 



