CH. XV] DEMONSTRATIONS OF NORMAL VISION 657 



The object should be right side up and face the condenser. If 

 the arc lamp is lighted there will be projected on the screen a sharp 

 image of the lantern slide. This image will be wrong side up as it 

 is in the eye. In order to have the image erect on the screen the 

 object must be wrong side up in the slide-carrier as in magic lantern 

 projection (fig. 8, 35). 



For the remainder of the experiments it is desirable to have the 

 screen image appear erect so that there may be no distraction from 

 the special points to be shown. It is worth while remembering 

 though, that when the image is wrong side up on the screen it will 

 be right side up in the eyes of the observers, and when it is right 

 side up on the screen it will be wrong side up in the eyes of the 

 observers. Objects appear right side up to a person only when the 

 image is wrong side up on his retina. 



921. Demonstration of the need of accommodation of the 

 eye for different distances of the object. It was pointed out by 

 Kepler in his discussion of vision that the eye as an optical instru- 

 ment could have a sharp image on the retina only in one position 

 of the object, unless some change took place in the eye. Every- 

 one with normal eyes knows that objects at all distances from 10 to 

 15 centimeters up to infinity can be seen with equal clearness. 

 Kepler thought that the power to see objects at different distances 

 was due to the possibility of changing the relative position of the 

 crystalline lens and the retina by the elongation and shortening 

 of the eye-ball. 



To demonstrate Kepler's hypothesis of accommodation there are 

 needed : 



(1) A3 diopter, convex, trial lens (fig. 384). 



(2) A lantern slide (fig. 383). 



(3) A white cardboard screen about half a meter (15 to 20 in.) 

 square to hold in the hands. 



If now the arc lamp is lighted and the lantern slide placed 36 to 

 37 centimeters from the lens a sharp image will be projected upon 

 the 5 meter screen. Now move the object to about 40 centimeters 

 from the lens; the image will not be clear, but much blurred. To 

 find the position of the sharp image, take the small white screen in 



