MEASURES OF ACUITY OF VISION 

 Measures of Acuity of Vision 



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From the theoretical standpoint the simplest measure of acuity or 

 " sharpness " of vision is the minimum angular separation which per- 

 mits of resolution for two point objects. As mentioned above, this 

 value for the normal eye is taken as 1 minute of arc. Hooke first 

 pointed out in 1671 that the resolving power of the normal eye for such 

 an object as a double star is about 1 minute of arc. The double star, 



Fig. 111-16. The upper letter E should be read by a normal eye at 50 ft. 

 lower line and upper letter C should be read at 40 ft. 



The 



imaged on the fovea of the retina, can be perceived as two point sources 

 if a relatively unstimulated cone lies between two others on which more 

 energy is received. 



Vision is usually tested at 6 meters or about 20 feet. For this pur- 

 pose a " page of type " is usually employed as a test object. In 1862 

 Snellen published a chart of test letters based on the assumption that 

 1 minute of arc is characteristic of the minimum separable. Figure 

 III— 16 illustrates the principle of construction of a typical test-chart 

 letter. Each letter in one row has a diameter subtending 5 minutes 

 of arc at a distance marked on the chart against the row, and the stroke 

 of the letter has a width subtending 1 minute of arc. A person with 

 normal vision should be able to read the letters of any row at the distance 

 indicated by the numerals set at the end of the row. The types are 

 printed for distances from 20 to 200 feet. The illumination should be 

 above 50 ft-candles, and glare must be avoided. 



