658 DEMONSTRATIONS OF NORMAL VISION [Cn. XV 



the hands and hold it in the path of the light from the lens. It 

 will be found at a point between two and three meters from the 

 lens. This shows that if the object is farther from the lens, the 

 image will be nearer to it. Conversely, if the object is brought up 

 toward the lens the image will move farther off. Kepler thought 

 that following the changes in the position of the sharp image 

 with change in the object, that for a near object the eyeball elon- 

 gated to bring the retina in the most favorable position, and that 

 when the object was far off the eyeball shortened to bring the retina 

 up to the point where the sharp image was formed. Such a method 

 of accommodation for objects at different distances would be 

 effective, as everyone knows who uses a photographic camera, but 

 as is now known it is not the method used by the eye of the higher 

 animals and man. 



922. Demonstration of Schemer's theory of accommodation. 



Scheiner admitted that the method of accommodation proposed 

 by Kepler would be effective, but he thought that the eyeball 

 remained unchanged in shape, and the crystalline lens changed its 

 shape, being more convex for near objects and less convex for 

 distant objects. He put it thus: "The crystalline lens of the eye 

 is equal to many glass lenses." 



There are needed for demonstrating Scheiner's theory : 



(1) A convex, trial lens of 3 diopters (fig. 382, 385). 



(2) A convex, trial lens of 4 diopters. 



(3) A lantern slide of fig. 383. 



Put the three diopter lens in the lens holder and light the arc 

 lamp. When the lens is 36 to 37 cm. from the lens a sharp image 

 will be projected on the 5 meter screen. Now move the object up 

 to 27 or 28 cm. from the lens. The _ 



image will be much blurred. Remove 

 the 3 diopter lens and put in its place 

 the 4 diopter lens. The screen image 

 will be sharp again. This shows that 

 if the crystalline can become more 

 and less convex, depending upon the 



position of the object, the screen image TION EXPERIMENT. 



