PRACTICAL EXERCISES 859 



needle is brought closer to the eye, and move away from each other 

 when it is moved away from the eye. If while the needle is in focus 

 one of the holes be stopped by the finger, the image is not affected. 

 When the eye is focussed for a greater distance than that of the 

 needle, stopping one of the holes causes the image on the other side 

 of the field of vision to disappear ; if the eye is focussed for a smaller 

 distance, the image on the same side as the blocked hole disappears 



(Fig- 331). 



4. Kiihne's Artificial Eye. This is an elongated box provided with 

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

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

 eosin 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- 

 tilled 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. 



FIG, 332. MAP OF BLIND SPOT (REDUCED BY ONE-HALF). 

 Right eye. Distance of eye from paper, 12 inches. 



(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, accommodation is not brought 

 about by a change in the distance of retina and lens, but by a change 

 of curvature in the lens. 



(b) 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 removed, for if 

 the latter be placed in front of the eye, the image is formed a little 

 behind the cornea. 



(f) Replace the lens. Move the retina so far back that the image 

 is focussed in front of it. This is the condition in the myopic eye. 



