THE BLIND SPOT 675 



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. 484. Diagram for Demonstrating the Blind Spot. 



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 disappear. 

 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. 



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



This area is large enough to cause a man completely to disappear at a dis- 

 tance 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 by some support; look with the right eye 

 at the top of the cross made on the left of the sheet 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. 



