PRACTICAL EXERCISES 



1061 



5. Kuhne's Artificial Eye. This is an elongated box provided with 

 a glass leps to represent the crystalline, and a ground -glass plate to 

 represent the retina. The box is filled with water to which a little 

 eosm has been added. The water must be perfectly clear. If the 

 tap-water is turbid it should be filtered or allowed to settle, or dis- 

 illed water should be used. A beam of sunlight or electric light, or, 

 in case these are not available, a beam from an oil stereopticon, is made 

 to pass through the box. Many of the facts of vision can be illustrated 

 by means of this piece of apparatus. The modification of it introduced 

 by Lyon is very convenient. 



(a) Let the rays of light pass through an arrow-shaped slit in a piece 

 of cardboard. An inverted image of the arrow is formed on the retina. 

 Move the retina nearer to or farther from the lens to make the image 

 sharp. In the eye of man and of most animals, accommodation is 

 not brought about by a change in the distance of retina and lens, but 

 by a change of curvature in the lens. 



(6) Remove the lens. The focus is now far behind the retina. This 

 illustrates the state of matters after the lens has been removed for 

 cataract. The arrow 

 can again be sharply 

 focussed on the retina 

 by putting a convex 

 lens in front of the 

 artificial eye. But 

 this must be much 

 weaker than the lens 

 which has been re- 

 moved, for if the 

 latter be placed in 

 front of the eye, the 

 image is formed a 

 little behind the 

 cornea. 



(c) Replace the lens. 

 Move the retina so 



Fig. 452. Schemer's Experiment. In the lower figure 

 the eye is focuased for a. point farther away than the 

 needle, in the upper for a nearer point. The con. 

 tinuous lines represent rays from the needle, the inter- 

 myopic eye. Put a rupted lines rays from the point in focus, 

 weak concave lens in 



front of the eye; the image now falls more nearly on the retina. Move 

 the retina forward so that the focus is behind it. This corresponds 

 to the hypermetropic eye. Put a weak convex lens in front of the 

 eye to correct the defect. 



(d) Observe that a plate with a hole in it, placed in front of the eye, 

 renders an indistinctly focussed image somewhat sharper by cutting 

 off the more divergent peripheral rays. 



(e) Fill with water the chamber in front of the curved glass that repre- 

 sents the cornea. The focus is now behind the back of the eye alto- 

 gether. Refraction by the cornea is here abolished, as is the case in 

 vision under water. An additional lens inside the eye, or a weaker 

 one in front of it, corrects the defect. Fishes have a much more nearly 

 spherical lens than land animals, and a flat cornea. 



(/) Fill the hollow cylindrical lens with water, and place it in front of 

 the artificial eye. The eye is now astigmatic. A point of light is 

 focussed on the retina, not as a point, but as a line. The vertical and 



