CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 663 



that it cannot be made more convex no matter how great the effort 

 at accommodation. If the eyes were originally normal, the images 

 of distant objects are still clear, but the diverging rays of near 

 objects can no longer be brought to a focus on the retina, without 

 artificial aid. 



For illustrating presbyopia there are needed : 



(1) A convex trial lens of 3 diopters (fig. 388). 



(2) A convex trial lens of i diopter. 



(3) A lantern slide of fig. 383. 



Place the 3 diopter lens in the metal holder and the lantern slide 

 in the slide-carrier, then move the slide up till it is 36 to 37 centi- 

 meters from the lens. Light the arc lamp and there will be a sharp 

 image on the screen. This represents the appearance for distant 

 objects. 



Now move the object up to 27 or 28 centimeters from the lens to 

 represent a near object. The image on the screen is much blurred, 

 something as the ordinary print of a newspaper looks to an old 

 man without spectacles. Put the i diopter lens with the 3 diopter 

 lens making a refracting medium equal to 4 diopters, and the image 

 on the screen will become clear and sharp, just as the print of the 

 newspaper becomes clear and sharp to the old man when he puts 

 on the proper spectacles. 



927. Astigmatism, Astigmia, or unequal curvature of a 

 refracting surface. This is a common defect in the eye, and is 

 found very frequently in the cornea. Roughly speaking an 

 astigmatic curve is like the bowl of a spoon or a hen's egg, the 

 curve being greater in one direction than in the direction at right 

 angles. 



The greater curvature will, of course, bring rays of light to a 

 focus sooner than the lesser curvature ; and with such a refracting 

 surface not all points in a circle can be focused sharply in any posi- 

 tion. Usually objects at right angles can be sharply focused by 

 changing the position of the objects, bringing them nearer for the 

 greater curvature and moving them farther away for the lesser 

 curvature. With a radial disc like fig. 389, 391, 392, if the vertical 

 lines are sharp in one position, the horizontal lines will be sharp in 



