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A MANUAL OF PHYSIOLOGY 



tion it is seen double. Then push it back slightly to the point at 

 which, again with maximum accommodation, it is just seen single. 

 Repeat the measurement with a needle mounted horizontally. If 

 regular astigmatism is present, the distances will not be the same. 

 Most eyes have slight regular astigmatism. 



In myopic persons the far-point of distinct vision can also be 

 determined by Schemer's experiment. The needle being left on 

 a shelf at the level of the eye, the person walks away from it back- 

 wards, regarding it all the time through the perforated card, till it 

 is no longer seen single. 



5. Ktihne'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- 



FIG. 435. SCHEINER'S EXPERIMENT. 



In the upper figure the eye is focussed for a point farther away than the needle, 

 in the lower for a nearer point. The continuous lines represent rays from the 

 needle, the interrupted lines rays from the point in focus. But the lines inside 

 the eye, which by an error in engraving are drawn as continuous lines, ought to 

 be interrupted, and vice versa. 



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. 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. 



(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 



