CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 665 



FIG. 391-392. RADIAL LINES IN BLACK AND WHITE FOR DETERMINING THE 

 PRESENCE OF ASTIGMATISM. 



A lantern slide of the radial lines is very desirable for the demonstrations on 

 astigmatism. 



The black lines on a white ground have the advantage that the lantern slide 

 is less liable to break than the white lines on a black ground ( 852). It will 

 be noticed that, by contrast, the central white circle seems lighter than the 

 white spaces between the radial lines. In like manner the central black circle 

 seems blacker than the black spaces between the radial lines. These are 

 optical illusions, for the white is uniform and so is the black. 



Now place the concave cylinder in front of the convex cylinder 

 and make their axes parallel (fig. 390). All the lines will become 

 sharp again. This is because the concave and the convex cylinders 

 with their axes parallel just balance each other and then act like a 

 piece of plane glass. To compare the effect of astigmatism on 

 printed matter with its effect on the radial lines, remove the cylin- 

 ders and focus sharply the lantern slide of the history of astigma- 

 tism, (fig. 393). Now add the 0.5 diopter convex cylinder and 

 make the axis vertical. The horizontal lines in the print will be 

 sharp, but the others blurred (fig. 394-395). Now rotate the 

 cylinder until its axis is horizontal, and the vertical lines will be 

 sharp and clear (fig. 396-397). It is to be noted that with these 

 Gothic letters, it is easier to read the words when the vertical lines 

 are clear, because vertical lines preponderate. 



