660 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV 



FIG. 386. TRIAL LENSES 

 FOR MYOPIA. 



924. Myopia, or short sight. My- 

 opia is due to any condition in the eye 

 by which the image of distant objects is 

 formed in front of the retina. The retina 

 is too far away, hence the retinal image is 

 blurred. Persons with this eye defect 

 are able to get clear images only when 



the object is quite close to the eyes. 



As the effort of accommodation only aids in seeing near objects, 

 there is no way by which short sighted persons can see distant 

 objects clearly without the use of a telescope or of concave spec- 

 tacles. 



For demonstrating myopia and its remedy are needed: 



(1) A convex, trial lens of 4 diopters (fig. 386). 



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



(3) A lantern slide of fig. 383. 



The 4 diopter lens is to represent the refractive power of the 

 myopic eye. It is placed in the metal lens holder, and the object 

 is brought up to a point 27 to 28 centimeters from it. Then the 

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



Move the slide back until the distance between it and the lens 

 is 36 to 37 centimeters. The image on the 5 meter screen will be 

 much blurred; it is too far off. One can prove this by taking the 

 white cardboard in the hands and finding the position of the sharp 

 image as in 92 1 . Now to get a sharp image on the 5 meter screen 

 it is necessary to reduce the curvature of the 4 diopter lens. Do 

 this by adding the i diopter concave lens. This reduces the 4 to a 

 3 diopter convex lens, and now the image is sharp and clear on the 

 5 meter screen. 



In the same way the short sighted 

 person can use concave spectacles which 

 will reduce the refractive power of the 

 cornea and the crystalline lens, and hence 

 the image will be formed farther away. 

 If the right spectacles are used, the image FJG ^ TRIAL LENSES 

 of distant objects will be clear and sharp FOR HYPEROPIA. 



