CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 66 1 



on the retina. If the person wishes to look at near objects the eye 

 is accommodated to make the crystalline more convex and the 

 diverging rays from near objects are brought to a focus on the 

 retina as with persons having normal eyes and not using spectacles. 



925. Hyperopia, or long sight. This eye defect is due to any 

 condition in which distant objects have their images formed 

 behind the retina. The retina is too close to the crystalline lens, 

 hence there is a blurred image formed on it even with parallel rays, 

 unless there is active accommodation and the crystalline lens is 

 made more convex. That is, with hyperopic eyes, no object can 

 be seen without effort. 



For the demonstration of hyperopia there are required: 



(1) A convex, trial lens of 2 diopters (fig. 387). 



(2) A convex, trial lens of i diopter. 



(3) A lantern slide of fig. 283. 



The lens of 2 diopters is to represent the refractive power of the 

 hyperopic eye. Place the lens in the metal holder, and light the 

 arc lamp. Move the object near to and distant from the lens, and 

 no place within the range will be found where a clear image will be 

 formed on the 5 meter screen. The screen is too near the lens and 

 the sharp image is formed somewhere behind it. If the room is 

 long enough the place where the image is sharp can be located. 



Now move the object until it is 36 to 37 centimeters from the 

 lens and add the i diopter convex lens. This will add its strength 

 and the refractive power will be equal to 3 diopters, and now the 

 image will be clear and sharp on the 5 meter screen. 



With the proper convex spectacles, the long sighted person can 

 see distant objects without effort, then when he wishes to see near 

 objects clearly the crystalline is made more convex as with normal 



eyes, thus making the entire range of 

 vision normal. 



926. Presbyopia, or old age sight. 



This comes gradually to every one with 

 advancing years, until finally, for most 



people after 60 or 6s years of age, the 

 FIG. 388. TRIAL LENSES J ^ . . 



FOR PRESBYOPIA crystalline lens has lost its elasticity so 



