THE BLIND SPOT 



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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 



FIG. 483. Disc of Concentric Lines for the Astigmatic Test. 



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

 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- 

 appear. This happens when its image falls on the retina directly over the 

 entrance of the optic nerve, which has no visual cells, and is, therefore, the 

 blind spot. This area is large enough to cause a man completely to 

 disappear from the field of vision at a distance of about one hundred 

 meters. 



Place a sheet of white paper at a distance of 30 cm. in front of the eye, 

 holding the head in a fixed position against the special support furnished; 

 look with the right eye at the top of the cross made on the left of the sheet 



FIG. 484. Diagram for Demonstrating the Blind Spot. 



of paper. Covering the sharpened portion of a lead pencil with white 

 paper, leaving the black tip exposed, move this pencil across the paper 

 from the visual center to the right. At a certain distance the black lead 

 will suddenly disappear. Mark this point. Continue to move the pencil 

 until the lead reappears. Mark this point. These two points represent 



