PRACTICAL EXERCISES 



991 



and a reading taken. Now rotate d through 90 degrees ; the long 

 axial lines of the images will be in alignment without further adjust- 

 ment. But if the eye is astigmatic, the short lines will not (Fig. 439) . 

 By rotating A, the short lines are made to coincide, so that a perfect 

 cross is again formed, and the graduation is read. The difference 



FIG. 438. 



FIG. 439. 



FIG. 440. PERIMETRIC CHART OF RIGHT EYE. 



Obtained with the perimeter shown in Fig. 415 (p. 946). The numbers represent 

 degrees of the visual field measured on the graduated arc of the perimeter. 



between this and the previous reading i.e., the difference between 

 the two pointers gives the difference in the curvature of the 

 cornea in the two meridians. The images of circles which form 

 the outer portion of the mires are oval in ordinary astigmatism. 

 7. Spherical Aberration. Close one eye, and bring a small object 



